Prepare the mushrooms by gently wiping each with a damp cloth and removing the stems. Set aside on a baking sheet lined with parchment paper.
In a skillet, heat olive oil over medium heat. Add minced garlic and sauté for about 1 minute until fragrant. Add chopped spinach and cook until wilted, about 2-3 minutes. Remove from heat and let cool slightly.
In a mixing bowl, combine softened cream cheese, shredded mozzarella, grated Parmesan, onion powder, salt, black pepper, and red pepper flakes. Mix until well combined. Stir in the sautéed spinach and garlic.
Stuff each mushroom cap with the spinach and cheese filling, packing it down gently. Preheat the oven to 375°F (190°C). Place stuffed mushrooms on a baking sheet in a single layer.
Bake in the preheated oven for about 20-25 minutes until mushrooms are tender and tops are golden brown. Let cool for a few minutes before serving.