01 -
Season the ground beef with salt and pepper. Divide the meat in half and form into 2 oval-shaped patties that will fit the bread slices. The patties should be slightly larger than the bread as they'll shrink when cooking.
02 -
Heat a large skillet over medium heat. Add the patties and cook until browned on both sides and cooked to your desired doneness, about 3-4 minutes per side for medium. Remove and set aside on a plate.
03 -
In the same skillet, add the olive oil. Add the sliced mushrooms and onions, cooking until they're softened and golden brown, about 7-8 minutes, stirring occasionally. Remove from the skillet and set aside.
04 -
Cut each patty in half. For each sandwich, place a beef patty half on a slice of rye bread. Top with a quarter of the sautéed mushrooms and onions and a slice of Swiss cheese. Place the other bread slice on top.
05 -
Wipe out the skillet and return to medium-low heat. Add half the butter and let it melt. Place two sandwiches in the skillet and cook until golden brown on the bottom, about 2-3 minutes. Add a bit more butter, flip the sandwiches, and cook until the other side is golden and the cheese is melted, about 2 minutes more. Repeat with the remaining sandwiches.
06 -
Slice each sandwich in half diagonally and serve immediately while warm and the cheese is still melty.