Divide the meat into 12 loose balls, then form into 1/4-inch-thick patties. Fold 1 slice cheese into a small square; center it on a patty, leaving a 1/2-inch border of meat around the cheese. Top with another patty. Crimp the edges of the 2 patties together, sealing the cheese inside. Repeat with the remaining cheese and meat to make 6 cheese-stuffed patties.
Warm the vegetable oil in a cast-iron skillet over medium-high heat. Add the onion and cook, stirring, until browned, about 10 minutes. Transfer the onion to a bowl and wipe out the skillet.
Add the patties to the skillet, season with salt and pepper, and cook 7 to 8 minutes. Flip, then poke the middle of each patty with a toothpick to release steam; cook 7 more minutes for medium well.
Put about 3 pickle slices on each bottom bun; top with the sauteed onion and a patty, then close with the top bun. Wait a few minutes before biting into the burger-the cheese is hot!
Photograph by Tina Rupp
Recipe adapted from Matt's Bar, Minneapolis for Food Network Magazine