Heat skillet to medium to medium high heat. I love iron skillet for these to get a good sear on them.
Slice onions into rings or half moon shapes, or can chop also. Saute on medium low heat in a tablespoon or so of butter, stirring occationally until soft and caramelized. When done, set aside on paper towel to drain.
Roll meat about golf ball size
Add 2 to 3 balls to hot skillet. Meat should start sizzling.
Let sear for 1 minute and flip over. Using spatula, flip over and smash patties until thinned and flat.
Cook until almost done. Flip once more and add Swiss to one patty and American cheese to another. Cover pan until cheeses melt.
For Frisco sauce just mix together the 2 dressings.
Spread softened butter on sourdough bread and grill or toast each side. Spread Frisco sauce on each slice.
Add caramelized onions and patties to toasted bread. Serve hot.