Instructions:

1. Begin by mixing the secret sauce ingredients in a small bowl. Adjust seasoning to taste, then set aside.
2. In a skillet over medium heat, melt 1 tbsp of butter. Add the thinly sliced onions, season with salt and pepper, and cook until caramelized, about 10-15 minutes. Transfer to a plate.
3. Form the ground beef into 4 patties, slightly larger than your bread slices. Season both sides with salt and pepper.
4. In the same skillet, cook the patties to your desired doneness, about 4 minutes per side for medium. Remove and set aside.
5. Butter one side of each bread slice. To assemble, spread the secret sauce on the non-buttered side of 4 slices, add a slice of cheese, a patty, caramelized onions, another slice of cheese, and top with another bread slice, buttered side up.
6. In a clean skillet over medium heat, grill the sandwiches until golden brown and the cheese is melted, about 3-4 minutes per side.

ENJOY!!