Remove the sausage from it’s casing and brown it in a large deep frying pan, over medium heat. Use a spatula to break up the sausage into small pieces as it browns.
Add ground fennel, dried oregano and red pepper flakes.
While the sausage is browning, finely chop one red bell pepper and add it to the sausage. Stir for about two minutes.
Next add chopped onion and chopped baby bella mushrooms to the mixture. Sauté over medium heat for about four minutes until the onions begin to soften and turn translucent, and the mushrooms loose most of their liquid.
Add minced garlic and sauté for about a minute, until it becomes fragrant.
Add salt and pepper to taste, keeping in mind that if you are using Parmesan, it will also add salt.
Transfer the sausage mixture to a large mixing bowl to cool, (unless you want a sausage omelet, you will want to get the temperature down a bit before adding the eggs.)
Preheat oven to 400 degrees.
Meanwhile, clean the portobellos with a damp paper towel, by wiping off any visible soil. Then using a spoon, scrape out the gills and remove any remaining stems.
Brush the tops of the caps with olive oil and then flip them onto their backs. They are ready to fill!
Add the chopped parsley, basil, and eggs to the sausage mixture. If you eat dairy, you can add grated Parmesan or Pecorino Romano cheese at this time.
Stir the stuffing mixture well, and evenly divide it among the mushroom caps. Then pack it down into the caps.
Place the caps on a parchment lined baking sheet and bake in a 400 degree oven for 15 minutes, or until the portobellos soften and the stuffing is heated through, and beginning to brown.