Divide beef into four balls and season with salt and pepper.
Heat skillet, place balls on parchment, and smash thin.
Cook until edges are crispy, then peel off parchment.
Flip, top with cheese, and cover to melt.
Remove patties and keep warm.
Sauté garlic in butter in the same pan.
Pour in cream, add Parmesan and parsley, and simmer until thickened.
Assemble burgers with lettuce, tomato, double patties, and onions.
Place burgers in the warm sauce and sprinkle with extra Parmesan.
Tip: Smash the patties as thin as possible for maximum crispy edges.