- Thoroughly combine beef, 1/3 cup roasted garlic teriyaki sauce and pepper; shape into 8 patties.
- Cook patties on grill 4 to 5 inches from hot coals 5 minutes.
- Meanwhile, sauté mushrooms in hot oil in large skillet over medium heat 4 minutes. Remove from heat and stir in remaining 1 Tbsp. roasted garlic teriyaki sauce.
- Turn patties over and cook 5 minutes longer, or until thoroughly cooked, topping with onion and cheese slices during last few minutes of cooking time.
- To serve, spread bun halves with mayonnaise; place burgers on bottom halves and spoon mushroom mixture equally over patties.