Credit: Albert Lynn via Flickr CC BY NC ND 2.0.

With sales reaching around 300 million per year, the Filet-O-Fish is one of McDonald’s most popular sandwiches.

But what many customers may not realize is that the sandwich was created by Catholics and for Catholics nearly 50 years ago.

In 1962, a man named Lou Groen had opened the first McDonald’s franchise in Cincinnati, Ohio. His restaurant was the first McDonald’s in the area, and it was not immediately successful.

Groen, who was once a homeless teenager, was working long hours to support his family and was becoming discouraged by the low profits.

At the time, the area was 87 percent Catholic, and the Church required the faithful to abstain from meat on Fridays all year round, not just during Lent.

Since McDonald’s had no meatless options on the menu, business was never good on Fridays.

Groen, himself a Catholic, decided to change that. He created a fish sandwich with a special batter and tartar sauce. He took the creation to headquarters, where McDonald’s chief Ray Kroc had also been working on a meatless patty – a pineapple slice on a cold bun, named the Hula Burger.

Kroc agreed that both items would be added to a test menu one Friday, and whichever sold more would become the permanent meatless addition.

In the end, it wasn’t even close. McDonald’s sold six Hula Burgers that day, and 350 Filets-O-Fish.

And with that, the Filet-O-Fish became the first ever addition to the original McDonald’s menu. The chain did change the recipe, however, switching the original halibut for cod in order to sell the sandwich more cheaply.

Groen said the Filet-O-Fish saved his franchise. He went on to own 43 McDonald’s restaurants in the region.