5 tablespoons butter, divided
1 large sweet onion, thinly sliced
1 1/2 pounds ground beef
3 tablespoons ketchup
2 tablespoons Worcestershire sauce
8 slices good quality bread (we used whole grain sourdough)
8 slices cheese (we used Swiss)
Salt and pepper, to taste
Make the caramelized onions: Heat 2 tablespoons butter in a large skillet to medium heat on the stove top. Add onions and stir to coat all of the onions with the butter. Cook, stirring every few minutes, until the onions have softened and turned a caramel color. The entire process should take about 15-20 minutes. Salt onions the last few minutes of cooking. Once onions are caramelized, transfer to a plate and cover with foil to keep them warm.
Make the burger patties: In a mixing bowl, combine ground beef, ketchup and Worcestershire sauce with your (clean) hands until just combined. Be careful not to overwork the meat or the patties will be tough when cooked.
Heat 1 tablespoon butter in the same skillet you cooked the onions in to medium-high on the stove top. When the buyer is melted and begins to bubble, add the patties. Cook for 6-8 minutes, flipping once in the middle of cooking. Once cooked, remove patties and place on a plate. Turn skillet down to medium heat.
Assemble the sandwiches: Butter one side of each slice of bread. Place one slice of bread butter side down on your work surface. Add two slices of cheese, then a burger patty. Top with a scoop of onions. Place another slice of bread on top of the sandwich, butter side out. Repeat for the other 3 sandwiches. Grill in your skillet until the cheese melts and bread begins to brown. Flip and grill the other side of the sandwiches.
Remove to a serving dish, slice and enjoy!