01 - Preheat the oven to 400°F. Line a baking sheet with parchment paper.
02 - Brush both sides of the Portobello caps with olive oil and season with salt and black pepper. Arrange gill-side up on the prepared baking sheet.
03 - In a skillet over medium heat, sauté chopped spinach until just wilted, about 2 minutes. Remove from heat and let cool slightly.
04 - Mix the cooled spinach with chopped artichoke hearts, cream cheese, sour cream, mozzarella, Parmesan, minced garlic, crushed red pepper flakes if using, salt, and black pepper until well blended.
05 - Spoon the spinach-artichoke filling evenly into each mushroom cap, pressing gently to fill.
06 - Sprinkle the stuffed mushrooms with shredded mozzarella and grated Parmesan cheese.
07 - Bake for 20 to 25 minutes, until mushrooms are tender and cheese is melted and golden brown.
08 - Allow to cool for 5 minutes before serving.