Preheat oven to 350 degrees with the oven rack in the middle.
Line a baking sheet with parchment paper, and add the portabella mushrooms with the opening up.
Preheat a large skillet over medium high heat. Add the olive oil and the sausage, breaking the sausage up with a wooden spoon. Cook for 5 minutes until the sausage is no longer pink.
While the sausage is cooking, add the mushroom stems, onion, bell pepper and garlic to a food processor. Pulse the vegetables until they are very finely chopped.
Add the finely chopped vegetable mixture to the italian sausage. Season with the salt and pepper. Cook another 3 minutes until the vegetables have softened and the sausage is cooked all the way through.
Remove the sausage mixture from the heat. Add in the parsley and stir.
Spoon the sausage mixture into the portabella mushrooms. Top each portabella with 2 tablespoons panko bread crumbs, then drizzle with olive oil.
Bake the stuffed mushrooms for 15 minutes until the mushrooms are tender. If necessary, turn your oven on broil to brown the panko bread crumbs for just 1-2 minutes.
Note: If your portabella mushrooms didn't come with stems, substitute 8 baby bella or button mushrooms. You don't want to skip it! The finely chopped mushrooms help bind the sausage mixture.
Leave off the panko bread crumbs for paleo, gluten free, low carb, Whole30.