Mix all secret sauce ingredients in bowl.
Form ground beef into 4 thin patties.
Season patties with salt and pepper.
Heat skillet over medium-high heat.
Cook onions until golden and caramelized.
Remove onions and set aside.
Cook patties 3-4 minutes per side.
Butter one side of bread slices.
Place bread butter-side down in skillet.
Spread secret sauce on unbuttered side.
Layer cheese, patty, onions, more cheese.
Top with second bread slice.
Cook until golden brown and crispy.
Flip carefully and cook other side.
Serve immediately while cheese is melted.