In a skillet over medium heat, melt a little butter and sauté the sliced onions until caramelized (about 15–20 minutes). Set aside.
In a bowl, season the ground beef with salt and pepper. Divide into 4 thin patties, slightly larger than your bread slices.
Cook patties in a skillet over medium-high heat until browned and cooked through. Set aside.
In a small bowl, whisk together all ingredients for the secret sauce. Set aside.
Spread butter on one side of each slice of bread. On the unbuttered side, layer cheese, a beef patty, caramelized onions, a spoonful of sauce, more cheese, and another slice of bread (buttered side out).
Grill each sandwich in a skillet over medium heat until the bread is golden and the cheese is melted, flipping once. Serve hot!