Remove the inside ribbing of the mushroom caps with a spoon. Brush olive oil on both sides of the mushrooms & season with salt and pepper.
Grill the mushroom caps until they've shrunk and are tender, about 5-7 minutes.
Remove from grill and with a tablespoon, spoon marinara sauce on inside of the mushroom then top with desired toppings and put on grill again and heat up until cheese has melted.
Serve hot, top with basil before serving.
Notes
You may use whatever toppings you desire. I just chose to use cheese and pepperoni. You can use diced bell peppers, onions, sausage, etc.