These Portobello mushrooms make a great side dish, or vegetarian option for your summer barbeque!
Heat your grill to medium and lightly oil a clean grill rack.
Drizzle the red onions and tomatoes with olive oil and season on both sides with salt and pepper. Grill the tomatoes and red onions for 2 to 3 minutes per side, until the tomatoes are softened and show grill marks and the onions are tender.
Drizzle the mushroom caps with olive oil and season both sides with salt and pepper. Grill the mushrooms, gill-side down, for 2 to 3 minutes. Turn the mushrooms over, evenly distribute the cheese over the mushrooms, and grill for 2 to 3 more minutes, until the cheese is melted and the mushrooms are cooked through.
Arrange the mushrooms on a serving platter and top each with a slice of tomato and a slice of grilled onion. Garnish each mushroom with a basil leaf and serve.