Prepare all of the ingredients. Preheat oven to 425deg.
In a large sauté pan over medium-high heat, add Italian sausage. Cook until you see no more pink and remove sausage from pan. Add bell peppers and onions and cook for 4-5 minutes.
In a bowl, combine all of the ingredients for the sauce.
In a 13x9 baking dish create the layers of the casserole as follows: ½ of sauce mixture, ½ of sausage, ½ of veggies, ½ of cheese, ½ of pepperoni, ½ of sauce mixture, remaining sausage, remaining veggies, remaining cheese, remaining pepperoni. Sprinkle top with parsley