Preheat oven to 375°F (190°C). Lightly grease a baking sheet.
Brush the mushroom caps with olive oil and place them gill-side up on the sheet.
In a skillet, sauté onion until soft. Add garlic and spinach, cooking until wilted.
Transfer the mixture to a bowl and stir in breadcrumbs, Parmesan, cream cheese, red pepper flakes, salt, and pepper.
Spoon the mixture into the mushroom caps, pressing gently to fill.
Bake for 20–25 minutes, until the mushrooms are tender and the tops are golden.
Sprinkle with fresh parsley before serving.