Brush both sides of each portobello mushroom cap with Italian dressing. Arrange mushroom on a baking sheet, gill sides up.
Bake mushrooms in the preheated oven until tender, about 12 minutes. Drain any juice that has formed in the mushrooms.
Beat egg, garlic, salt, and black pepper together in a large bowl.
Stir spinach, pepperoni, Parmesan cheese, 3 tablespoons mozzarella cheese, and 3 tablespoons bread crumbs into the eggs until evenly mixed.
Divide spinach mixture over mushroom caps; sprinkle mushrooms with remaining 1 tablespoon mozzarella cheese and 1 tablespoon bread crumbs. Return mushrooms to the oven.
Continue baking until topping is golden brown and cheese is melted, about 10 minutes more.
