This burger was born in Oklahoma out of Depression-era resourcefulness: In the 1920s, cooks at El Reno’s Hamburger Inn used onions to stretch hamburger meat. That location has closed, but you can try a version at other El Reno restaurants like Johnnie’s Hamburgers & Coneys, Sid’s Diner and Robert’s Grill. To make one yourself, smash at least half an onion into each patty.
Preheat the oven to 250˚ F. Mix the beef with 1 teaspoon each salt and pepper in a large bowl, then form into 8 balls.
Heat the vegetable oil in a large skillet over medium-high heat. Add 2 of the beef balls and slightly flatten with a metal spatula. Place a handful of the sliced onions on each patty (you’ll use about a quarter of the onions for 2 patties — it will seem like a lot!) and flatten the beef into 1/4-inch-thick patties, pressing the onions into each burger. Cook until browned on the bottom, about 3 minutes. Carefully flip the burgers along with the onions and cook until the onions are browned and tender, about 2 more minutes. Flip the burgers again, along with the onions, and top each with a slice of cheese. Cover and continue cooking until the cheese melts, 30 seconds to 1 minute. Transfer to a baking sheet and keep warm in the oven until serving.
Repeat with the remaining beef and onions to make 8 burgers total, adding more vegetable oil to the skillet if needed. Serve 2 burgers on each bun with ketchup, mustard and/or pickles.