Steam the cauliflower. Place cut cauliflower in a microwave safe bowl with 1 cup of water. Cover with a damp paper towel and microwave on high for approximately 3 minutes or until the cauliflower is tender. Don’t overcook!
Drain the cauliflower pat dry with a paper towel. Set aside.
Preheat oven to 400 degrees F.
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.
In the same skillet at olive oil and saute mushrooms over medium high heat for 10 minutes to remove the excess water. Set aside when done.
Prepare a 13x9 inch casserole dish by spraying it with non-stick spray then spread ½ cup of pasta sauce on the bottom.
In a large bowl add the cauliflower, cooked Italian sausage, mushrooms, and green peppers. Toss together until well mixed.
Spread ½ of the mixture in the casserole dish. Top with ½ cup of pasta sauce, followed by ½ of the pepperonis (kind of press them down in between the other toppings), and finally 6 ounces of mozzarella cheese.
Next spread the remaining toppings mixture over the cheese followed by the pepperonis (save 10 slices for the top of the casserole), remaining ½ cup of pasta sauce, and 6 ounces of mozzarella cheese.
In a small bowl mix together the Parmesan cheese and Italian seasoning.
Sprinkle the Parmesan mixture over the casserole and top with 10 slices of pepperoni.
Place in oven and bake for 30 minutes, or until the casserole is warmed through and the cheese is fully melted.