Combine meat, salt and pepper. Shape to form 16 patties, about ½-inch thick.
Cook on ridged grill pan or in skillet preheated on medium-high heat 3 to 4 minutes per side or until internal temperature is 160°F. Top each patty with a slice of Swiss cheese.
Meanwhile, heat 1 Tbsp. olive oil in separate large skillet on medium-high heat. Add onion and cook 2 minutes, stirring frequently. Add 1 pkg. mushrooms and cook, stirring frequently, for 4-5 minutes until lightly browned; remove to a plate. Add remaining oil and mushrooms to skillet. Cook, stirring frequently, for 4-5 minutes until lightly browned. Add Worcestershire sauce and onion-mushroom mixture; stir.
Spread ½ Tbsp. mayonnaise on bottom of each bun. Place patties on buns; top each with onion-mushroom mixture. Sprinkle about 1 Tbsp. bacon on each and close with bun tops.
Tip: Use tip of handle of wooden spoon to make hole in center of each patty for uniform cooking. The hole will close as the patties cook.