Preheat the oven to 375°F.
In a skillet, heat the olive oil over medium heat. Add the minced garlic and sauté for about 1 minute until fragrant.
In a large bowl, combine the cooked Italian sausage, ricotta cheese, mozzarella cheese, Parmesan cheese, parsley, Italian seasoning, salt, and black pepper. Mix well.
Stuff each mushroom cap generously with the cheese and sausage mixture.
Place the stuffed mushrooms on a baking sheet lined with parchment paper.
Bake in the preheated oven for 20-25 minutes, or until the mushrooms are tender and the tops are golden brown.
Remove from the oven and let cool for a few minutes before serving.