Lightly spray a large nonstick pan with vegetable cooking spray and place over medium high heat. Add in the peppers, zucchini, mushrooms, and onions. Saute until the onions are clear, about eight minutes. Remove from heat and stir in the olives and basil.
Lightly spray vegetable cooking spray on one side of each slice of bread and place spray side up on a baking sheet. Broil until golden brown, about one minute. Remove bread from broiler but leave broiler on.
Spread mustard on bread, top with two tomato slices, cooked vegetables, and sprinkle evenly with mozzarella cheese. Place back in broiler until cheese is melted, about one minute.