Preheat your oven to 350 degrees F (175 degrees C) and grease a 13 by 9-inch glass baking dish with extra-virgin olive oil.

Bring a large pot of water to a boil over high heat, add the cavatappi pasta and 2 tablespoons of kosher salt, and cook until al dente, about 7 to 9 minutes. Drain and set aside.

In a large skillet over medium heat, add 1 tablespoon of olive oil, then add the Italian sausage and cook until browned and cooked through, about 4 to 5 minutes. Transfer to a bowl.

In the same skillet, add sliced mushrooms and another tablespoon of olive oil, cooking until browned and tender. Season with a little salt and add to the sausage bowl.

Add chopped onion and minced garlic into the skillet with the last tablespoon of olive oil and cook until onions are soft, about 5 to 7 minutes.

Stir in the tomato sauce, diced tomatoes, dried oregano, basil, crushed red pepper flakes, and the remaining salt. Reserve 2 cups of this sauce in a separate bowl.

Fold in the chopped green bell pepper, cooked sausage, mushrooms, and reserved pasta into the remaining sauce in the skillet.

Layer half of the sausage and pasta mixture in the prepared baking dish, followed by 1 cup of shredded mozzarella cheese, then the remaining sausage mixture, and finally pour the reserved tomato sauce over everything.

Cover the dish with foil and bake for about 20 minutes until hot and bubbling.

Remove the foil, sprinkle with remaining mozzarella cheese and sliced pepperoni, and bake for another 15 to 20 minutes until cheese is melted and pepperoni is browned.

Let the casserole sit for a few minutes before serving.