01 - Preheat 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 the skillet and sauté until wilted, about 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, combine the spinach-artichoke mixture with cream cheese, mozzarella, Parmesan, salt, black pepper, and crushed red pepper flakes if using. Mix until fully blended.
06 - Spoon the filling evenly into each mushroom cap, mounding slightly. Sprinkle with extra Parmesan cheese.
07 - Bake for 22 to 25 minutes or until mushrooms are tender and the filling is golden and bubbly.
08 - Remove from the oven and allow to cool for 5 minutes before serving.