Grilled Portabella Mushrooms 3 portabella mushrooms 4 Tbsp. low sodium vegetable broth 3 Tbsp. chopped onion 4 cloves garlic, minced 4 Tbsp. balsamic vinegar Clean mushrooms and remove stems. Place caps on a plate with the gills up. In a small bowl, combine the broth, onion, garlic and vinegar. Pour mixture evenly over the mushroom caps and let stand for 1 hour. Grill on a preheated George Foreman grill for 10 minutes. Serve immediately. Makes 3 servings.