Prep Time: 10
Cook Time: 10
Serves: Serves: 4
Shape ground beef into 4 patties, 4 inches in diameter and 1/2-inch thick. Preheat grill or broiler. Grill patties, 5 minutes per side or to desired doneness, adding one slice Cheddar and one slice Provolone cheese during last minute of cooking.
Meanwhile, heat oil in skillet and cook pepper, onion and mushrooms until tender.
Serve burgers in focaccia topped with vegetable mixture.