Gather ingredients and set grill to medium heat.
Mix together balsamic vinegar, basil, oregano, garlic and salt and pepper and pour into the inside of each mushroom cap.
Let mushroom caps marinate for 15 minutes, brush grill grate with olive oil and then place them on the heated grill.
Grill for 5-10 minutes and then add cheese and grill for an additional 2 minutes.
Place buns on grill for the last minute of grilling.
Remove mushrooms and buns from grill and plate.
Top with lettuce, tomato, onions and avocado (or your own desired toppings.)
Top it with a toasty bun and enjoy!