01 - Preheat oven to 375°F and line a baking sheet with parchment paper.
02 - Clean mushroom caps and gently remove stems. Finely chop stems and set aside.
03 - Heat 2 tablespoons olive oil in skillet over medium heat. Add chopped onion and cook until softened, approximately 3 minutes.
04 - Add garlic and chopped mushroom stems to skillet. Sauté for 2 minutes until fragrant.
05 - Stir in chopped spinach and cook until wilted, about 2 minutes. Remove from heat.
06 - Transfer spinach mixture to bowl. Add cream cheese, Parmesan, breadcrumbs, salt, pepper, and red pepper flakes if using. Mix until well combined.
07 - Fill each mushroom cap generously with spinach mixture and arrange on prepared baking sheet.
08 - In small bowl, combine 2 tablespoons breadcrumbs, 2 tablespoons Parmesan, and 1 tablespoon olive oil. Sprinkle mixture over stuffed mushrooms.
09 - Bake for 20-25 minutes until mushrooms are tender and tops are golden brown.
10 - Let cool for a few minutes before serving.