1. Preheat your grill or grill pan over medium heat.
2. In a mixing bowl, combine olive oil, balsamic vinegar, salt, and pepper. Brush the mixture onto both sides of the portobello mushroom caps.
3. Place the mushrooms on the grill, gill side down, and cook for about 5 minutes. Flip and cook for another 5 minutes until tender.
4. Toast the burger buns on the grill for a minute or two until lightly browned.
5. Assemble the burgers by placing a grilled mushroom cap on the bottom half of each bun. Top with tomato slices, red onion slices, and lettuce leaves. Cover with the top half of the bun.