In 1926, Henry Wesly Davis opened a Route 66 hamburger stand, the Hamburger Inn, in El Reno, Oklahoma. He had the idea from White Castle hamburgers to added a handful of thin sliced white onions to a small hamburger patty and created the Fried Onion Burger. The Hamburger Inn is closed, but El Reno celebrates the El Reno Fried Onion Burger Day Festival on the first Saturday each May.
In 1937, John Steinbeck recorded how Al made the burger at Al and Susy's Place, in El Reno. While the meat was cooking on the grill, Al pressed the onions into it with a spatula and grilled a split bun. He buttered and added pickled relish to the bottom bun before assembling the burger. Ross Davis, Henry Davis's son, insist there should not be butter, just pickle relish and mustard. - AsNotedIn