2. In a mixing bowl, combine olive oil, balsamic vinegar, salt, and pepper. Brush the mixture onto the portobello mushrooms.
3. Place the mushrooms on the grill, gill side down, and cook for about 5-7 minutes. Flip and cook for another 5-7 minutes until tender.
4. Toast the whole grain bread slices on the grill for about 1-2 minutes per side.
5. Assemble the sandwich by placing a grilled portobello mushroom on a slice of toasted bread. Top with a slice of provolone cheese, tomato, lettuce, and the other slice of bread.