Cook the sausage. In a large skillet over medium high heat cook the Italian sausage, approximately 15 minutes. When the sausage has finished cooking drain the excess fat. Set aside 

Prepare casserole dish. Prepare a 13 x 9 inch casserole dish by spraying it with non-stick spray then spread ½ cup of pasta sauce on the bottom

Toss together. In a large bowl add the cauliflower, cooked Italian sausage, mushrooms, and green peppers. Toss together until well mixed.

Spread the mixture. Spread ½ of the mixture in the casserole dish. Top with ½ cup of pasta sauce, followed by ½ of the pepperonis and finally 6 ounces of mozzarella cheese.

Spread remaining mixture. Spread the remaining toppings mixture over the cheese followed by the pepperonis, remaining ½ cup of pasta sauce, and 6 ounces of mozzarella cheese.

Prepare cheese mixture. In a small bowl mix together the Parmesan cheese and Italian seasoning. Sprinkle the Parmesan mixture over the casserole

Bake casserole. Place in the oven and bake at 400 degrees F. for 30 minutes, or until the casserole is warmed through and the cheese is fully melted.