Heat oil in large skillet. Sauté ground beef, onion and garlic over medium-high heat until browned. Add zucchini, mushrooms and green pepper. Cover and simmer for 5 to 7 minutes until vegetables are tender. Stir in tomato sauce, basil, thyme, salt and pepper. Simmer uncovered until most of the liquid has evaporated. Serve on toasted buns. Can freeze for future use.