Directions
1. Place Portobello mushrooms, olive oil, balsamic vinegar, minced shallots, thyme and garlic in a bowl. Toss mushrooms; marinate for 2-3 hours.
2. Preheat a grill to medium-high (450–550°F).
3. Grill mushrooms for 2-3 minutes per side or until mushrooms are tender.
4. Remove mushrooms from direct heat. Return mushrooms to top rack of grill. Top with a slice of Brie. Close lid to barbeque and continue to cook until brie melts (approximately 2-3 minutes).
5. Remove from grill and place on buns.
6. Garnish with caramelized onions.