One of the longest-running and most successful sandwiches at McDonald’s is the Filet-O-Fish. But it wasn’t always on the menu. It was created by an entrepreneurial Catholic who had a big problem and won a legendary wager.

The story goes back to 1962. Lou Groen had opened up the first McDonald’s franchise in Cincinnati, Ohio, but wasn’t able to make a profit. He quickly pieced together why: His restaurant was located in a Catholic neighborhood (Groen himself was also Catholic), Catholics didn’t eat meat on Fridays (all year round, at the time), and McDonald’s only served entrées with meat.

He quickly got to work on a solution. He created a special batter, made his own tarter sauce, and put it together into a new kind of fish sandwich.

But since he was franchising, he couldn’t start selling it on his own. So he took his new creation to McDonald’s headquarters and showed it to Ray Kroc, the CEO of the company.

Kroc wasn’t sold on Groen’s sandwich right away. Part of the reason was that Kroc had been working on his own meatless sandwich: it was like a normal burger but had a slice of pineapple instead of meat, and he called it the Hula Burger.

Kroc made Groen a wager: they would make Groen’s fish sandwich and Kroc’s Hula Burger available for one Friday at one location, and whichever one sold more would be added to the national menu.

“Friday came and the word came out. I won hands down,” Groen later recalled. “I sold 350 fish sandwiches that day. Ray never did tell me how his sandwich did.”

And the rest is history: Groen’s fish sandwich was added to the national menu as the Filet-O-Fish and has been there ever since.

“My fish sandwich was the first addition ever to McDonald’s original menu,” Groen explained. “It saved my franchise.”

And, boy, did his operation grow after that! When Groen finally sold his franchise 24 years later, he had grown his business to 43 McDonald’s restaurants.

[See also: Your Comprehensive Guide to Ash Wednesday Ashes]

[See also: Everything You Need to Know About Lent Explained in 60 Seconds]