Slice 2 large yellow onions thinly and melt 2 tablespoons of butter in a skillet over medium-low heat. Add the onions, stirring occasionally, and cook for 25-30 minutes until they turn golden brown and jammy.

In a mixing bowl, combine ½ cup of mayonnaise, 2 tablespoons of ketchup, 1 teaspoon of Dijon mustard, 1 teaspoon of Worcestershire sauce, and ½ teaspoon of smoked paprika. Whisk until well blended.

Take 1 pound of ground beef and shape it into thin, wide patties. Season both sides with salt and pepper.

Heat a skillet or griddle over medium-high heat. Cook the patties for about 3-4 minutes on each side until browned and cooked through.

Butter one side of each slice of rye bread, layer with Swiss cheese, cooked beef patty, caramelized onions, more cheese, and another slice of bread with butter facing out.

Grill the assembled sandwiches in the heated skillet for about 3-4 minutes on each side, pressing gently with a spatula.

Once grilled to perfection, remove the sandwiches from heat, slice in half, and serve hot with extra secret sauce on the side.