Nevertheless, Utahns are fiercely attached to their fry sauce. They demand it nearly everywhere they travel.

"It's kind of a funny thing to go out of state and ask for fry sauce. They look at you like you're crazy," Thompson said.

Though there are numerous variations of fry sauce, the basic recipe is simple — one part ketchup, two parts mayonnaise. Many restaurants then spike this with salt, spices, garlic, relish, horseradish, even pickle juice. It's mostly used for dunking fries, though some restaurants are equally happy to slather it on burgers.

"Once you've had it, you've always got to have it," said Chrissy Riley, who moved to Salt Lake City 11 years ago from Little Rock, Ark. She was enjoying two cups of the sauce with her fries during a recent Crown Burger visit.

At a nearby booth, Utah native Jake Robertson sat with a cup of ketchup and another of fry sauce. He says his sister, who moved to Richmond, Va., has resigned herself to making fry sauce at home.

"She obviously can't find it anywhere," he said.

Though regional alternatives to ketchup abound — barbecue and hot sauce in the South, malt vinegar in the Northeast, a mix of mayonnaise and mustard in Hawaii — few condiments have taken hold of the collective consciousness of a region the way fry sauce has in Utah.

Just look at Roberts' filing cabinets: They are filled with thousands of letters from people who no longer live in Utah asking for his fry sauce. Sometimes, Mormon missionaries abroad claim to go through fry sauce withdrawal.

Roberts honors each request — for free — because he likes receiving letters. He gives away about 25 bottles a week.

"They all have a story to tell. We sent one (bottle) as a wedding gift to someone in California because her husband loves fry sauce so much she wanted to give him a bottle for his wedding night," he said.

It's all evidence, he says, that fry sauce holds a special place in Utah's culture.

Even more than the state's official snack food of green Jell-O?

"I think even more so. I think we have fry sauce specifically in Utah. People really correlate that with Salt Lake City," Roberts said. "Jell-O is sold in every state in the nation. You can't say green Jell-O is synonymous with Utah."