Pre heat a grill pan or outdoor grill. Remove stem from each mushroom and brush outside to remove any dirt. Do not wash your mushrooms! Whisk together olive oil and vinegar. Brush the mushrooms with oil/vinegar and season with salt and pepper. Place mushrooms on the grill and cook on each side for about 5 minutes until tender. Top with cheese for the last minute of grilling. Place on bun with onion, lettuce, tomato and mozzarella. Serve and enjoy!