In a skillet, melt 1 tablespoon of butter over medium heat and add sliced onions.
Cook, stirring occasionally until onions are tender, but not mushy.
Remove from skillet and keep warm.
Use remaining softened butter, butter one side of each slice of bread and set aside.
Form ground beef into 2 patties slightly larger than a bread slice.
Heat skillet to medium-high heat and cook meat patties until desired doneness is reached. Set aside.
Work quickly so the meat does not get cold, wipe out the skillet and place back on medium heat.
Add 2 slices of bread to the skillet with the buttered side down.
Place a meat patty on top of each slice of bread.
Top each meat patty with 2 slices of Swiss cheese.
Top each with half of the onions and the last slices of bread, butter side up.
Leave in the skillet until bread is lightly toasted.
Carefully flip the sandwich and toast the other side.
Remove from skillet.
Serve with patty melt sauce if desired.
Notes
Any ground beef can be used. 80/20 makes a juicier burger patty.
I prefer marbled Rye bread for appearances.
Bread can be toasted in a seperate pan for convenience. Leave the burger patty in the skillet and top with the cheese and onions, cover with a lid and heat until the cheese is melted.
How to Make Patty Melt Sauce - Combine 4 tablespoons mayonnaise with 2 tablespoons French dressing, 2 tablespoons sweet pickle relish, 1/4 teaspoon of onion powder, 1/4 teaspoon garlic powder, and a pinch of black pepper. Allow it to sit while you are making the patty melts. If you don’t have French dressing, ketchup will do in a pinch. Store leftover sauce for up to 1 month in the refrigerator. Great on regular burgers too!