Pre-heat your oven to 350 degrees. In a small saucepan, add spinach, artichoke hearts, and 1 cup of water. Bring to a boil, then reduce heat and simmer for 3-5 minutes. White the spinach and artichokes are simmering, cut the cream cheese into cubes and warm in the microwave for about 1 minute. Remove spinach and artichokes from heat, and drain really well. Return the spinach and artichokes to the pan, and with the heat on low, stir in cream cheese, sour cream, mayo, and parmesan cheese. Continue stirring until all ingredients are incorporated. Stir in garlic powder, salt, black pepper, and red pepper flake if desired. On a rimmed baking sheet, lay out the mushrooms. Scoop a large spoonful of artichoke and spinach mixture into each mushroom. Bake for 15-20 minutes, then serve immediately. You may have extra dip leftover, either stuff more mushrooms, or enjoy it with fresh baguette or crackers.