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.