Combine the ingredients and refrigerate until ready to use.
For the melts
Heat the oil in a large skillet over medium heat.
Add the onion and caramelize, cooking for about 20 minutes, stirring often. The onions should turn a nice golden brown. Remove from the pan.
Divide the beef into 4 equally-sized patties. Cook them as desired in a skillet or on the grill.
Add 1 slice of Swiss cheese and 2 slices of American cheese to each patty and let melt.
Spread butter on one side of each slice of bread. Place in a large skillet over medium heat and cook until the butter has melted and the bread is golden crispy.
Make sandwiches by spreading Frisco sauce on the unbuttered sides of the bread. Add the patties and onions and top with another piece of bread. Serve.