Can you build a better burger? It turns out you can—and should. Lately, the all-American classic is getting a striking makeover, especially when it comes to the fixings, which are getting more adventurous, more gourmet, and more interesting.

In the quick-service category, where burgers appear on 36 percent of menus, that means thinking outside the bun about toppings, sauces, and condiments. By including toppings such as premium cheeses like smoked Gouda and Gorgonzola—these cheeses are up 23 percent on burger menus since 2013, according to a Datassential report on burger trends—and ethnic flavors like the ever-popular sriracha, restaurants are raising the stakes on one of the most universal menu items.

At Des Moines, Iowa–based Zombie Burger, which has two locations, that means stunt burgers piled high with everything from chicken-fried bacon to peanut butter. Between the regular menu and the special burgers of the week, Zombie has designed more than 200 burgers, like the Raygun—topped with Monterey Jack cheese, fried jalapeños, caramelized onion, bacon, guacamole, and chipotle mayo—and the Machete Kills Again, a Spanish-inspired burger that incorporated breaded and fried cheese enchiladas topped with pork carnitas, white queso, and guacamole.

Inspiration comes from other cuisines and pop culture, too. There have been burgers that riffed on Star Wars, the presidential election (the Monte Christie, a beer-battered and fried triple bacon cheeseburger named after New Jersey governor Chris Christie, appeared during the Iowa caucus), and The Walking Dead.

“We do pride ourselves on a great cheeseburger,” says Tom McKern, executive chef. “But as we started to grow, we noticed that our customer base really wanted stunt burgers as well.” The tamer specials just don’t sell as well, he adds.

While Zombie Burger’s clientele might be seeking a specific extreme experience, they do reflect a broader desire among consumers for more adventurous burgers. One way that trend appears in burger chains across the country is heat—and plenty of it.

At Washington, D.C.–based Burger Tap & Shake, each table is stocked with a bottle of sriracha. “That’s a huge component for us,” executive chef Jeff Tunks says.

Meanwhile, at Dugg Burger, which has locations in Dallas and Plano, Texas, and specializes in customizable burgers with a hollowed-out top bun, myriad toppings appeal to spice lovers.

“Customers really like hot menu items, so we’ve been doing a lot of pepper things, like ghost pepper cheese spread. We had a hatch chile week in September,” says Jeff Braunstein, managing partner at Dugg.

The trend toward heat has also asserted itself in the form of spicy Korean barbecue sauce and spicy Thai peanut sauce. For customers who “want to be more adventurous but aren’t there yet,” Braunstein says, he and his team experiment with familiar flavors, presenting always-popular guacamole spiked with chimichurri and whipping up bloody Mary ketchup (a New Year’s Day special).

Dugg offers 12 standard toppings—lettuce, grape tomato, dill pickle, Dugg sauce (a souped-up Thousand Island), ketchup, mustard, barbecue sauce, sautéed mushrooms, fire-roasted jalapeños, caramelized onions, crispy onion strings, and hickory-smoked bacon—alongside a rotating topping called the “Lucky 13,” which changes every day or two.

“It’s where we can try to attract people who are willing to try something new,” Braunstein says. On a given day, customers might find fried eggs, chipotle ranch, Lucky Charm marshmallows melted down with habanero (“The really adventurous people will try it,” he says), or balsamic Brussels sprouts (“Our general manager said it was probably the most popular overall”). And, of course, bacon is a reliable favorite: For a recent bacon week, Dugg had prepared bacon jam, candied bacon, bacon-cheese spread, bourbon-bacon slaw, and bacon aioli.

Indeed, from the bacon aioli at Dugg to the cilantro raita at Burger Tap & Shake, sauces are going way beyond ketchup and mustard, which is on-trend; Datassential found that 53 percent of consumers expressed interest in savory spreads. Balsamic vinegar is adding its tang to tartar sauce at Boston-based Wahlburgers, while there’s pesto at Michigan’s Moo Cluck Moo. Ranch is a crowd-pleaser at Dugg, where it’s a frequent Lucky 13—sometimes in variations like chipotle ranch—and Zombie Burger, where it graces the Trailer Trash Zombie (also topped with chicken-fried bacon and cheese curds).

Even the most basic of savory spreads, mayonnaise, is prime for revision.

“We’re all about the mayo here,” says Max Albano, corporate chef at Good Stuff Eatery, the fast casual founded by chef Spike Mendelsohn and based in Washington, D.C. “Everyone dipped their fries in something when they were younger.”

Good Stuff stocks a self-service dip bar with four mayos for personalized dunking: mango, chipotle, sriracha, and Old Bay. Those are all in addition to the specialty mayos that top the burgers themselves; the Prez Obama burger is topped with Applewood bacon, onion marmalade, Roquefort cheese, and horseradish mayo, while the Michelle Melt—a free-range turkey burger with caramelized onions, Swiss cheese, ruby tomato, and lettuce—is dressed with Southlawn Herb Garden Mayo.

Simple or elaborate, fixings are getting upgraded right in-house. “People are looking for artisanal components for all of their toppings,” says Burger Tap & Shake’s Tunks. “So we take a real homemade approach to everything.”

The fast casual offers an all-purpose sauce, “AP sauce,” made with ketchup, mayonnaise, mustard, chipotle, and barbecue sauce, that’s so popular, customers often buy it by the pint to go. Tunks says the brand might explore bottling and selling it in the future.

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At Moo Cluck Moo, a two-unit operation in Michigan, homemade aioli is a major part of the burgers’ flavor profiles, from the cumin aioli on the Baa Burger (ground lamb topped with Feta and sliced onions) to the steakhouse aioli on the Steak Burger (1⁄3 pound ground beef topped with Swiss cheese and sautéed onions and mushrooms).

Cofounder Brian Parker says that as Moo Cluck Moo was growing, the team made a conscious decision to avoid off-the-shelf product. “Most of it is too processed for us,” he says. “Some of the big sourcing companies, because we were so small, didn’t want to talk to us, so it worked in our favor. [We thought,] ‘We can do this on our own!’”

Wahlburgers takes a similar approach, making elevated versions of basic condiments like barbecue sauce, Wahl sauce (a homemade sauce with a sriracha base), onion jam, and tomato jam in-house.

“We’re cooks,” executive chef Paul Wahlberg says. “We take pride in making so much stuff in the restaurants.” Of course, Wahlberg also knows that sometimes customers want what’s familiar. “I’ve worked in places where you make your own ketchup, but people are like, ‘Where’s the regular ketchup?’ They want Heinz.”

Perhaps that’s the same kind of nostalgia factor that makes comfort-food flavors like macaroni and cheese and Frito pie play so well at Dugg. “I think people want that safe, warm food that takes them back to when they were a child,” Braunstein says.

That doesn’t mean that customers aren’t also game to deviate from the standard lettuce-tomato-pickle flavor profile in favor of more advanced toppings, though.

“Pickled items are very popular,” Wahlberg says. Wahlburgers offers pickled onions and vegetables, while pickled veggies adorn the vegetarian Haight-Ashbury falafel burger at Burger Tap & Shake. Roasted flavor notes are big, too: Wahlburgers dresses its Thanksgiving Burger with a roasted butternut squash spread, and Dugg Burger has roasted everything from habanero chilies to artichokes and celery root.

They might sound more like side dishes than burger toppings, but Braunstein says that diners’ low-carb conscientiousness, plus Dugg’s dug-out bun, translate into vegetable-based Lucky 13 toppings performing very well. “You tend to feel you’re eating a healthier meal when you’re getting a burger with those toppings,” he says.

It seems that customers value variety, too: Many quick-service burger establishments offer specialty burgers that rotate weekly or monthly. From the over-the-top creations at Zombie Burger to smaller riffs on the classic burger, these specials supplement the regular menu and inspire devotion among customers.

“It really gives us a boost at the beginning of the month, because they’re like, ‘Oh my god, I had that sandwich last year,’” Parker says of Moo Cluck Moo’s revolving specials, called Craveables. “I can’t tell you two months ago what the Craveable was, but people are like, ‘Remember that one you had in October? That was amazing.’”

Sometimes, as in the case of the Baa Burger, a really popular Craveable will land on the permanent menu. Likewise, at Burger Tap & Shake, some of the composed burgers on the menu—like the Southern Comfort, a beef burger with pimento cheese, fried green tomato, Vidalia onions, and bread-and-butter pickles—began as burgers of the month, then became so popular they were upgraded. Pre-selected topping combinations not only work well as a rec ipe of flavors and textures—because chefs created them—but they also help with efficiency.

“People like the option, and it helps us expedite service,” Tunks says. “It makes their decision-making a little easier.”

It also proves that the basic burger is a blank culinary canvas, lending itself to iterations that might riff off Southern cuisine, Vietnamese banh mi, or Korean barbecue, inspiring endless creativity in the kitchen. Just don’t abandon the originals: Good Stuff’s Albano says that the classic hamburger, cheeseburger, and bacon cheeseburger comprise 35 percent of the concept’s weekly sales.

Customers are getting more adventurous—especially younger ones—but still, Braunstein says that Dugg’s customers tend to build the same burgers over and over. “It’s just a creature of habit,” he says.