Place peppers, onion and mushrooms in food processor and blend until fine. Place beef in large bowl, add veggies and mix well. Form into 4 patties. Grill over medium heat 5-6 minutes per side, or until cooked through. Top with cheese and transfer to buns.
Other ways to sneak more veggies into your foods: spaghetti and pizza Sauce, smoothies, meatloaf, soups and even brownies!