Step 1

Preheat oven to 350 degrees F (175 degrees C).

Step 2

Brush both sides of each portobello mushroom cap with Italian dressing. Arrange mushroom on a baking sheet, gill sides up.

Step 3

Bake mushrooms in the preheated oven until tender, about 12 minutes. Drain any juice that has formed in the mushrooms.

Step 4

Beat egg, garlic, salt, and black pepper together in a large bowl.

Step 5

Stir spinach, pepperoni, Parmesan cheese, 3 tablespoons mozzarella cheese, and 3 tablespoons bread crumbs into the eggs until evenly mixed.

Step 6

Divide spinach mixture over mushroom caps; sprinkle mushrooms with remaining 1 tablespoon mozzarella cheese and 1 tablespoon bread crumbs. Return mushrooms to the oven.

Step 7

Continue baking until topping is golden brown and cheese is melted, about 10 minutes more.