Grilled veggie sandwiches are a summertime staple for many vegetarians. If you don't have a grill, broil the veggies in your oven instead.
Put the vinegar, garlic, salt, pepper, and 4 Tbsp. olive oil in a small jar and shake well. Set aside.
Clean the mushrooms and remove their stems. Brush the vegetables with olive oil. Grill the mushrooms for 3-4 minutes on each side. Place the onion and tomato chunks on skewers and grill, turning often, until lightly charred. Cut each mushroom in half, and cut the onion and tomato chunks into thin slices.
Spread a little of the vinegar mixture on each slice of bread, then top with the mushrooms, onions, tomatoes, and basil leaves.
Makes 4 servings.