I've been very leery about trying the whole substituting mushroom for beef idea but after trying this burger I'll never make another beef or "veggie burger" again.
4 portobello mushroom caps
1/4 cup balsamic vinegar
2 tablespoons olive oil
1 teaspoon dried basil
1 teaspoon dried oregano
1 tablespoon minced garlic
salt and pepper to taste
4 (1 ounce) slices provolone cheese
baby spinach leaves
Place the mushroom caps, smooth side up, in a shallow dish. In a small bowl, whisk together vinegar, oil, basil, oregano, garlic, salt, and pepper. Pour over the mushrooms. Let stand at room temperature for 15 minutes or so, turning twice.
Preheat grill for medium-high heat.
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.
Top with tomato and spinach