Add olive oil to a large pan set to medium heat. Brown Italian sausage for 3-4 minutes.
Add onions, mushrooms, riced cauliflower, salt, pepper, and Italian seasoning until vegetables are mostly tender, about 5 minutes.
Add tomato paste and broth. Stir well to combined. Simmer filling for 8-10 minutes under low-medium heat.
Stir in parmesan cheese. Adjust seasoning if needed.
Slice peppers in half (lengthwise) and scoop in filling. Top with mozzarella cheese and bake for 20-25 minutes until tops are golden brown. Garnish with fresh basil.