Prepare the Sauce:

In a small bowl, combine the mayonnaise, Dijon mustard, ketchup, garlic powder, black pepper, and Worcestershire sauce. Mix well and set aside.

Cook the Onions:

In a skillet over medium heat, melt 1 tablespoon of butter. Add the sliced onions and cook, stirring occasionally, until they are caramelized and golden brown, about 10-15 minutes. Remove from the skillet and set aside.

Prepare the Patties:

In a large bowl, season the ground beef with salt and black pepper. Divide the beef into four equal portions and shape each into a patty.

In the same skillet over medium-high heat, melt the remaining tablespoon of butter. Cook the patties for about 3-4 minutes per side, or until they are cooked to your desired level of doneness. Remove from the skillet and set aside.

Assemble the Sandwiches:

Toast the Texas toast or bread slices in a toaster or in the oven until they are golden brown.

Spread a layer of the creamy pepper sauce on each slice of toast.

Place a slice of American cheese and a slice of Monterey Jack cheese on four of the slices.

Top with a beef patty and some of the caramelized onions.

Place the remaining toast slices on top to complete the sandwiches.

Grill the Sandwiches:

In the same skillet over medium heat, grill each sandwich for about 2-3 minutes per side, or until the bread is crispy and the cheese is melted.