2. In a mixing bowl, combine olive oil, balsamic vinegar, and garlic powder. Brush the mixture onto both sides of the portobello mushroom caps.
3. Grill the mushrooms for 5 minutes on each side, or until tender.
4. Assemble the sandwiches by placing a grilled mushroom cap on the bottom half of each bun. Top with arugula, tomato slices, and the top half of the bun.