2. In a bowl, mix olive oil, balsamic vinegar, and garlic powder. Brush the mixture onto both sides of the portobello mushrooms.
3. Place the mushrooms on the grill, gill side down. Grill for about 5 minutes per side, until tender.
4. Assemble the sandwiches by placing a grilled mushroom on the bottom half of each bun. Top with tomato slices, lettuce, and cheese if using. Cover with the top half of the bun.