Preheat your oven to 475 degrees. Prepare a rimmed baking sheet with parchment paper. Set aside.
Wash and wipe the Portobello mushroom caps with a damp paper towel to remove any dirt. Using a spoon, scrape out and remove the gills from the underside of the mushroom caps.
Adjust your oven racks to upper middle and lower middle positions. You will be using both racks.
Combine oil, salt, and pepper in a large resealable bag.
Adjust until blended.
Add mushrooms.
Adjust mushrooms until coated and allow to sit at room temperature for at least 30 minutes but preferably an hour.
Add 2-3 tablespoons of pizza or pasta sauce to the inside of each cap. Spread sauce around with a brush if possible.
Sprinkle each cap with Italian seasoning and then add cheese, approximately ¼ cup per cap. Place pepperoni and additional pizza toppings on top of cheese.
Bake for between 7-10 minutes or until the cheese melts. Broil mushrooms for a minute or two, if desired, to brown cheese and other toppings. Enjoy!
Place your mushrooms gill side down on top of your parchment paper.Bake on the lower rack until tender, about 20 minutes.Flip mushrooms gill side up.Carefully stuff each mushroom with filling, as evenly as possible. Both your pan and mushrooms will be hot.Bake on upper rack until stuffing is a nice golden brown color, between 3 and 5 minutes.Serve and enjoy every bite!