01 - Preheat oven to 375°F. Line a baking sheet with parchment paper.
02 - Place cleaned mushroom caps stem-side up on the prepared baking sheet.
03 - Heat olive oil in a skillet over medium heat. Add minced garlic and cook until fragrant, about 30 seconds.
04 - Add chopped spinach to the skillet and sauté until wilted, approximately 2 minutes. Stir in chopped artichoke hearts and cook for an additional minute. Remove from heat and allow to cool slightly.
05 - In a mixing bowl, combine the spinach-artichoke mixture with cream cheese, mozzarella, Parmesan, salt, black pepper, and red pepper flakes if using. Mix thoroughly until well incorporated.
06 - Spoon the filling evenly into mushroom caps, mounding it slightly. Sprinkle each filled cap with the extra Parmesan cheese.
07 - Bake mushrooms for 22 to 25 minutes, until tender and the filling is golden and bubbly.
08 - Remove from oven and allow to cool for 5 minutes prior to serving.