Preheat oven to 375F. Clean mushrooms with a slightly damp paper towel, remove the stems and scrape out the gills. Place mushroom caps top side down, on a baking sheet lined with parchment paper. Brush the tops with olive oil and bake for 5 minutes.
Remove from the oven and increase oven temperature to 425F. Flip the mushroom caps over and soak up any liquid with paper towels. Add the marinara sauce, part of the mozzarella cheese, sausage, pepperoni and top with the remaining cheese.
Bake again at 400F for an additional 10 minutes, or until cheese is melted and golden. If the cheese isn’t melted enough, bake for another 2 minutes on the top rack of your oven. Allow to cool slightly before serving. Garnish with a little sweet basil and enjoy!