01 - Set oven to 375°F and 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 skillet and sauté until wilted, approximately 2 minutes. Stir in chopped artichoke hearts and cook for an additional minute. Remove from heat and let cool slightly.
05 - In a mixing bowl, blend the spinach-artichoke mixture with cream cheese, mozzarella, Parmesan, salt, black pepper, and red pepper flakes if using, until thoroughly combined.
06 - Spoon the filling evenly into each mushroom cap, mounding slightly. Sprinkle with extra grated Parmesan cheese.
07 - Bake filled mushrooms for 22 to 25 minutes, or until tender and the filling is golden and bubbly.
08 - Remove from oven and allow to cool for 5 minutes prior to serving.