In a greased cast-iron or other heavy skillet, saute onion and garlic for 1 minute. Add the zucchini, yellow squash and carrots; saute until tender. Drain and cool to room temperature., In a large bowl, combine the egg, bread crumbs, tuna, salt and pepper. Add vegetable mixture. Shape into six 3-1/2-in. patties., In same skillet, cook patties in butter until lightly browned, 3-5 minutes on each side. Serve on buns with cheese, lettuce and tomato.