4 tablespoons butter, divided
1 large or 2 medium yellow onions, thinly sliced
1 1/2 pounds ground chuck
1 envelope dry onion soup mix
4 to 8 slices Swiss cheese
8 slices rye or pumpernickel bread
Ketchup and mustard (optional)
Add 2 tablespoons butter to a skillet. Add the onions, and cook over low heat for about stirring for 15 to 20 minutes, stirring occasionally. Remove from heat and set aside.
In a large bowl, mix ground beef with onion soup mix until just combined. Do not over mix. Form into 4 patties.
Preheat grill pan or large skillet. Add burger patties. Cook burgers a few minutes on one side and then flip and cook on the other side for another 2-3 minutes or until done to your liking.
To assemble the patty melts, place a burger on one slice of bread. Top with 1 to 2 slices of cheese and top with some caramelized onions. Top with another slice of bread. Repeat to make 4 sandwiches.
Spread the outsides of the bread with remaining butter. Grill sandwiches in a skillet pressing down with a spatula and grill until golden brown, about 2-3 minutes. Flip and grill on the other side. Cut in half and serve with ketchup and mustard if you like.