Grilled Portobello Mushrooms

Ingredients
3 portobello mushroom caps
1/4 cup olive oil
1/2 medium onion, chopped
3 cloves garlic, minced
4 tablespoons balsamic vinegar
Mozzarella cheese slices (optional)

Directions
Briefly rinse the mushrooms and drain, or wipe the mushrooms with a damp cloth.

Place caps in a shallow bowl or baking dish, without overlapping, gill side up.

In a small bowl, combine the oil, onion, garlic and vinegar. Pour the mixture over the mushroom caps. Let stand for at least an hour. Grill over a hot grill for about 10 minutes. During the last minute or two on the grill, top with slices of mozzarella cheese if desired.

Serve burger-style with lettuce and tomatoes.