01 - In a shallow dish, combine balsamic vinegar, olive oil, minced garlic, salt, and black pepper. Whisk gently to blend.
02 - Place portobello mushroom caps in the marinade, gill side up. Spoon marinade over each cap. Allow to marinate for 30 minutes at room temperature, turning once halfway.
03 - Preheat grill to medium-high heat.
04 - Arrange marinated mushrooms on the grill, gill side down. Grill each side for 5 to 7 minutes, until mushrooms are tender and charred.
05 - Place grilled mushrooms onto burger buns and add preferred toppings. Serve immediately.