Season the ground beef with salt and pepper, and form into 4 patties.

In a skillet over medium heat, melt 1 tablespoon of butter. Add the patties and cook until browned on both sides and cooked to your desired doneness. Remove and set aside.

In the same skillet, add the olive oil and sliced onion. Cook until the onion is caramelized, about 10 minutes. Remove and set aside.

Butter one side of each slice of rye bread. On the non-buttered side, place a slice of Swiss cheese, a beef patty, some caramelized onion, another slice of Swiss cheese, and top with another slice of bread, buttered side up.

Cook the sandwiches in the skillet over medium heat until the bread is toasted and the cheese is melted, about 3-4 minutes per side.