Preheat the Oven: Preheat your oven to 375°F (190°C).
Prepare the Mushroom Caps: Clean the mushrooms with a damp paper towel and remove the stems. Set aside.
Sauté the Spinach: In a skillet, heat the olive oil over medium heat. Add the minced garlic and sauté for about 1 minute until fragrant. Add the chopped spinach and cook until wilted, about 2-3 minutes. Remove from heat.
Mix the Filling: In a mixing bowl, combine the sautéed spinach, cream cheese, mozzarella cheese, Parmesan cheese, salt, pepper, and red pepper flakes. Mix until well combined.
Stuff the Mushrooms: Spoon the filling into each mushroom cap generously.
Bake: Place the stuffed mushrooms on a baking sheet and bake for 20-25 minutes, or until the mushrooms are tender and the filling is golden.
Garnish and Serve: Remove from the oven, garnish with fresh parsley, and serve warm.