Portobello mushroom burgers

1. Place the mushroom caps, smooth side up, in a shallow dish. In a small bowl, whisk together vinegar, 2 tablespoons olive oil, basil, oregano, garlic, salt, and pepper. Pour over the mushrooms. Let stand at room temperature for 15 minutes or so, turning twice. While mushrooms are marinating, chop onions. Preheat skillet for medium-low heat with 1 tablespoon olive oil. Place onions in skillet and cook for about 20 minutes, until onions are caramelized.

2. Preheat grill for medium-high heat.

3. Brush grate with oil. Place mushrooms on the grill, reserving marinade for basting. Grill for 5 to 8 minutes on each side, or until tender. Brush with marinade frequently. Top with cheese during the last 2 minutes of grilling. Add caramelized onions on top of cheese.