Preheat your oven to 400°F (200°C). Place the cleaned portobello mushroom caps gill-side up on a baking sheet. Lightly brush the inside and outside of the mushroom caps with olive oil, then sprinkle with salt and black pepper.
In a small bowl, combine the pizza sauce, dried oregano, and garlic powder. Spoon approximately 2 tablespoons of the seasoned pizza sauce into each mushroom cap, spreading it evenly. Top each mushroom generously with shredded mozzarella cheese, then add the mini pepperoni (if using).
Bake for 20-25 minutes, or until the mushrooms are tender and the cheese is bubbly and golden brown. Remove from the oven, sprinkle with grated Parmesan cheese, and serve hot. For a crispier finish, you can broil for the last 1-2 minutes, watching carefully to prevent burning.
Notes
For a vegetarian option, simply omit the pepperoni and add diced bell peppers, onions, or sliced black olives instead. These mushrooms can also be grilled for a smoky flavor – just cook over medium-high heat until tender and cheese is melted.