In 1962, Lou Groen was desperate to save his floundering hamburger restaurant, the first McDonald’s in the Cincinnati, Ohio, area. His problem: The clientele was heavily Catholic. Back then, most Catholics abstained from meat every Friday, not just during Lent, a 40-day period of repentance that began last week with Ash Wednesday. His solution: He created a sandwich that would eventually be consumed at a rate of 300 million a year — the McDonald’s Filet-O-Fish. “Frisch’s (the local Big Boy chain) dominated the market, and they had a very good fish sandwich,” recalled Groen, now 89. “I was struggling. The...