Preheat oven to 375 degrees. Place a large skillet over medium heat and add oil to the pan. Then add in onion and garlic and cook, stirring occasionally for 2 minutes. Add in chopped chicken sausage, cauliflower rice and mushrooms.
Cook, stirring occasionally for 4-6 minutes or until cauliflower rice is tender. Add in marinara sauce and toss to coat. Season with salt and pepper to taste.
Remove the tops and seeds from the bell peppers and place them in a 13 x 9 pan. Scoop the stuffing mixture into each bell pepper, filling each one.
Spoon a little extra marinara on top of each one if desired. Bake for 35-40 minutes or until bell peppers are tender.