With a paper towel or clean dry cloth, wipe off any dirt or debris from your mushrooms. Then cut off the stems and scrape out the gills with a spoon.
Line your baking tray with parchment paper and place the mushrooms on the sheet pan, stem side down. Lightly brush or spray the mushroom caps with a little oil.
Place the mushroom caps in the oven for 10 minutes.
Take the mushrooms out of the oven and allow them to sit for about 5 minutes. Then use a paper towel to soak up any excess moisture on or around the mushrooms.
Flip the mushrooms over and fill them with 2-3 tablespoons of pizza sauce, any pizza topping that you wish, (I used vegan pepperoni and spinach), then top them with vegan cheese.
Return them to the oven for another 10 minutes until the cheese is melty. (You can turn on the broiler for an additional 2-3 minutes to melt the cheese better.
Notes
Some recipes call for salting the mushrooms before baking, but I find that they get too watery when I do this, so I salt them after baking if needed.
Some vegan cheese does not melt as well as others, to help it melt, you may need to cover the stuffed mushrooms with foil for about 5 minutes trapping the steam and helping the cheese to melt.