Preheat your oven to 375°F (190°C). Line a baking sheet with parchment paper.
Clean the mushrooms with a damp paper towel and remove the stems. Set the mushroom caps aside.
Chop the mushroom stems finely. Heat olive oil in a skillet over medium heat and sauté chopped onion for 2 minutes.
Add the garlic and chopped mushroom stems. Cook for another 2-3 minutes.
Stir in the spinach and cook until wilted. Remove from heat.
In a large bowl, combine the spinach mixture, cream cheese, Parmesan, feta, salt, pepper, and red pepper flakes.
Spoon the mixture into each mushroom cap, pressing it in slightly. Sprinkle with breadcrumbs if using.
Bake for 18–20 minutes until the mushrooms are tender and the tops are golden brown.
Serve warm and enjoy!