Portobello Mushroom Burgers

2 Portobello mushrooms
3 tablespoons balsamic vinegar
1 tablespoon olive oil
1 teaspoon minced garlic
1/2 teaspoon pepper
1/4 teaspoon salt
Gorgonzola Cheese
Mozzarella Cheese

Cook on the grill for 5 - 8 minutes per side.