Add the olive oil to a large rondeau or frying pan over medium-high heat and sear the sausage on all sides until golden brown. These do not have to be cooked through.
Set the sausage to the side on a plate and add the onions to the pan sauté for 2 to 3 minutes.
Add in the mushrooms and continue to sauté for 4 to 5 minutes or until lightly browned and tender. See notes below.
Stir in the peppers and sauté for 3 to 4 minutes or until lightly browned.
Mix in the garlic just until fragrant, which takes about 1 minute.
Deglaze with the marsala wine and cook until it is almost gone, or au sec.
Pour in the beef stock and cook for 5 to 6 minutes to infuse and concentrate the flavor.
Season the liquid with salt and pepper and add back in the seared sausage and cook until the sausage is warmed up and cooked throughout, which takes about 4 to 5 minutes.
Serve the sausage and peppers over some polenta or risotto and garnish with parmesan cheese and optional chopped fresh parsley.
