01 -  Place cauliflower florets in a large microwave-safe bowl with 1 cup of water. Cover with a damp paper towel and microwave on high for about 3 minutes until just tender. Do not overcook. Drain well and pat dry with paper towels. Set aside. 
 02 -  Preheat oven to 400 degrees Fahrenheit. 
 03 -  In a large skillet over medium-high heat, cook the sausage, breaking it up with a wooden spoon, until browned and cooked through, about 15 minutes. Drain excess fat and set aside. 
 04 -  In the same skillet, add olive oil and sauté mushrooms over medium-high heat for 8 to 10 minutes until they release and evaporate their moisture. Set aside. 
 05 -  Spray a 13 by 9 inch casserole dish with non-stick spray, then spread 1/2 cup of pasta sauce over the bottom. 
 06 -  In a large mixing bowl, combine steamed cauliflower, cooked sausage, sautéed mushrooms, and green bell pepper. Toss until well mixed. 
 07 -  Layer half of the mixture into the prepared casserole dish. Top with 1/2 cup pasta sauce, half of the pepperoni slices, pressing some between layers, and 6 ounces of shredded mozzarella cheese. 
 08 -  Layer the remaining vegetable and meat mixture over the cheese. Top with the remaining 1/2 cup pasta sauce, most of the remaining pepperoni, reserving 10 slices for topping, and the remaining 6 ounces mozzarella cheese. 
 09 -  In a small bowl, mix together Parmesan cheese and Italian seasoning. 
 10 -  Sprinkle the Parmesan mixture evenly over the casserole and top with the reserved 10 slices of pepperoni. 
 11 -  Bake in the preheated oven for 30 minutes, or until the casserole is bubbling and the cheese is golden and melted. 
 12 -  Allow to stand for 5 minutes before serving.