Start by cutting the cauliflower into small florets and steam them in a microwave-safe bowl for about 3 minutes. Once soft, drain thoroughly and pat dry with a paper towel.

In a large skillet over medium-high heat, cook the Italian sausage for about 15 minutes or until browned. Drain any excess fat.

In the same skillet, add the chopped mushrooms and sauté over medium heat for around 10 minutes until golden and moisture has evaporated.

Preheat your oven to 400°F (200°C). In a greased casserole dish, spread ½ cup of pasta sauce across the bottom. Combine the steamed cauliflower, cooked sausage, sautéed mushrooms, and diced green peppers. Layer half this mixture in the casserole dish, followed by more sauce, pepperoni, and mozzarella cheese.

Repeat the layering process with the remaining mixture, topping with sauce, remaining pepperoni, mozzarella cheese, and a sprinkle of parmesan cheese and Italian seasoning.

Place the assembled casserole in the preheated oven and bake for about 30 minutes, until the cheese is melted and golden brown.

Let the casserole cool for 10-15 minutes before serving to allow the layers to set.