Preheat oven to 350F.

Clean mushrooms by removing stems and gills. Chop the stems and set aside for now.

Add the hot sausage and onion to dutch oven or heavy pot over medium heat and cook until browned, adding a swish of oil if needed. Make sure to break up the meat as much as possible. Turn off the heat, then stir in the garlic, chopped bell pepper and mushroom stems.

Place the mushrooms caps on a parchment-lined sheet pan then spoon the sausage mixture into the mushroom caps.

Bake for 18 - 20 minutes at 350F.

Top with cheese if desired and broil for 1-2 mins or until cheese melts.

Move the mushrooms onto a paper towel-lined plate to dry them off a bit.

Garnish with basil if desired and serve.