Preheat your broiler on high. Set an oven rack in the middle of the oven. Line a rimmed broiler-safe baking sheet with foil for easy cleanup.
Wipe the mushrooms clean with a damp paper towel. Gently twist off the stem of each mushroom. Holding each mushroom in one hand, use a spoon to gently scrape out the gills.
Spray the mushroom caps with olive oil spray on both sides, and sprinkle them with kosher salt, black pepper, and garlic granules. Broil until just tender, about 4 minutes on each side.
Remove the mushrooms from the oven, keeping the broiler on. Drain them, gill side down, on paper towels. Wipe the baking sheet dry, then return the mushrooms, gill side up, to the baking sheet.
Fill each mushroom cap with half of the marinara sauce. Top with cheese and pepperoni (or your favorite toppings).
Place the mushrooms back under the broiler, in the middle of the oven, just until the cheese is melted, about 2 minutes. Serve immediately.
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Notes
Since the mushrooms shrink quite a bit when we pre-broil them, it's best to buy large mushrooms - as large as you can find. It's also best to use flat mushrooms, to help mimic a regular pizza crust.Even if you did everything right, the mushrooms might still release some water. Simply tilt each mushroom carefully to drain the water and place them on paper towels for a few minutes. They'll still be delicious!