Preheat the oven to 375°F (190°C).
Clean the mushrooms and remove the stems.
Finely chop the stems and set aside.
Cook the sausage in a skillet over medium heat until browned.
Add the chopped mushroom stems and garlic to the skillet and sauté until soft.
In a bowl, combine the sautéed mixture with ricotta, Parmesan, parsley, salt, pepper, and red pepper flakes.
Fill each mushroom cap generously with the stuffing mixture.
Drizzle olive oil over the stuffed mushrooms before baking.
Bake in the preheated oven for 20-25 minutes until the mushrooms are tender and the tops are golden.
Let the stuffed mushrooms cool slightly before serving.