2. In a mixing bowl, combine olive oil and balsamic vinegar. Brush the mixture onto the portobello mushrooms.
3. Grill the mushrooms for about 5-7 minutes on each side, until tender.
4. Assemble the sandwiches by placing a grilled mushroom on the bottom half of each bun. Top with arugula, tomato slices, and provolone cheese. Cover with the top half of the bun.
Notes:
Feel free to add your favorite condiments or extra toppings like avocado or caramelized onions.