In a small bowl, whisk together mayonnaise, ketchup, relish, Worcestershire sauce, garlic powder, and paprika. Refrigerate until needed.

Heat 2 tbsp butter and 1 tbsp olive oil in a skillet over medium-low heat. Add onions and a pinch of salt. Cook slowly for 20-25 minutes until caramelized and sweet. Remove and set aside.

Divide ground beef into 4 equal portions and form into thin patties matching the shape of your bread. Season with salt, pepper, and onion powder.

Heat a large skillet over medium-high heat. Cook the patties for 3-4 minutes per side until a crust forms and beef is cooked through. Remove and wipe the skillet clean.

Spread the secret sauce on one side of all 8 slices of bread. On 4 slices, layer Swiss cheese, a beef patty, caramelized onions, and Cheddar cheese. Top with the remaining bread, sauce side down.

Spread softened butter on the top, outside-facing slice of each sandwich.

Grill the sandwiches butter-side down in the skillet over medium-low heat. Butter the top side facing up while the bottom cooks.

Cook for 4-5 minutes per side until the bread is golden brown and crispy, and the cheese is fully melted. Slice in half and serve hot.