1. Lightly brush each portobello mushroom with olive oil and season with salt and pepper. Place mushrooms gill side up on grill over medium-high heat. Cook for 4-5 minutes and then flip mushrooms. Cook for another 4-5 minutes.
2. Place provolone cheese slices on top of each mushroom. Cook until cheese is melted. Place hot mushrooms on sandwich buns. Top with condiments of your choice-tomato slices, red onion, lettuce, mustard, ketchup, etc. Serve warm.