Portobello Mushroom Burgers

So, you just invited your vegetarian friend over to grill, what to serve, what to serve?  This sounds good to me and I am a big fan of red meat but I like portobellos too.

4 portobello mushroom caps
1/4 C. balsamic vinegar
2 tbsp. olive oil
1 tsp. dried basil
1 tsp. dried oregano
1 tbsp. minced garlic
salt and pepper to taste
4 slices provolone cheese

1. 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.
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.