Heat a large skillet over medium heat. Add the sausage and cook for 10 to 12 minutes. Transfer the sausage to a plate.
In the same skillet, add oil and bring to medium-high heat. Add green bell pepper and saute three minutes. Add onion, saute another minute. Add garlic and saute one more minute.
Add the tomatoes, water, spaghetti, Italian seasoning, salt and red pepper flakes, if using. Stir to combine. and bring the mixture to a boil. Add pepperoni and sausage. Cover and reduce the heat to medium-low and cook for about 15 minutes, or until the spaghetti is cooked.
Stir in the Parmesan cheese until melted. Sprinkle mozzarella cheese on top before serving. Garnish with chopped parsley.
Recipe reprinted with permission but slightly adapted from Incredible One Pot Cooking.I needed to add another 1/4 cup of water to the mixture while pasta was cooking to make sure there was enough liquid. Keep an eye on the liquid level and adjust as needed.