Preheat the oven to 375°F (190°C).
In a mixing bowl, combine the cooked sausage, pizza sauce, mozzarella cheese, Parmesan cheese, oregano, garlic powder, and red pepper flakes. Mix until well combined.
Place the mushroom caps on a baking sheet, hollow side up. Spoon the filling mixture into each mushroom cap, packing it slightly.
Bake in the preheated oven for 20-25 minutes, or until the mushrooms are tender and the cheese is bubbly and golden.
Remove from the oven and let cool for a few minutes. Garnish with fresh basil leaves if desired before serving.