01 - Preheat oven to 375°F and line a baking sheet with parchment paper.
02 - Arrange 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 and sauté until wilted, about 2 minutes. Stir in chopped artichoke hearts and cook for an additional minute. Remove from heat and let mixture cool slightly.
05 - In a mixing bowl, blend 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 mushroom caps, mounding slightly, then sprinkle with extra Parmesan cheese.
07 - Bake for 22 to 25 minutes until mushrooms are tender and filling is golden and bubbly.
08 - Remove from oven and allow to cool for 5 minutes before serving.