Preheat oven to 375°F. Line a baking sheet with foil.
Wipe mushroom with a damp cloth or paper towel, and scoop out gills and stem. Put gill side down on the baking sheet, and bake for 10 minutes.
Meanwhile, stir together cream cheese, ¼ cup Parmesan cheese, garlic, oregano, basil, red pepper flakes, and salt. Stir in bell pepper, onion, and sun-dried tomato.
Take mushroom out of oven and wipe off. Flip over and fill mushroom with cheese mixture.
Bake for 10 minutes. Sprinkle with remaining ¼ cup cheese, and bake another 5 minutes. Serve with pizza sauce if desired. Enjoy!