Preheat your oven to 400°F (200°C) to ensure even cooking of the casserole.

In a microwave-safe bowl, steam cauliflower florets with a splash of water for approximately 3 minutes until tender, then drain and pat dry.

In a medium skillet, cook the Italian sausage for about 15 minutes until browned, breaking it apart with a spatula as it cooks.

Sauté sliced mushrooms in the same skillet for about 10 minutes until golden and tender, then set aside.

Grease a 13x9 inch casserole dish and spread ½ cup of marinara sauce evenly across the bottom.

In a large bowl, combine the steamed cauliflower, cooked sausage, sautéed mushrooms, and chopped green peppers, mixing until well combined.

Layer half of the mixture in the greased casserole dish, topping with half of the remaining sauce, half the pepperoni, and half the mozzarella cheese, then repeat.

Sprinkle the top with mixed Parmesan cheese and Italian seasoning, then bake in the oven for 30 minutes until the cheese is melted and bubbling.

Allow the casserole to rest for 10–15 minutes before slicing and serving to enhance texture and flavor melding.