Pizza Casserole

Name: Pizza Casserole
Prep time: 20 min
Total time: 20 min
Cuisine: American
Ingredients: 4 tbsp. extra-virgin olive oil, divided, plus more for pan; 12 oz. cavatappi pasta; 2 tbsp. plus 2 tsp. kosher salt, divided; 1 lb. Italian sausage, casings removed; 8 oz. cremini mushrooms, stemmed and sliced; 1 small yellow onion, chopped (about 1 1/2 c.); 2 cloves garlic, finely chopped; 3 (8-oz.) cans tomato sauce; 1 (14.5-oz.) can diced tomatoes; 1 1/2 tsp. dried oregano; 1 tsp. dried basil; 1/4 tsp. crushed red pepper flakes; 1/2 green bell pepper, seeds and stem removed, chopped into 1/2" pieces; 3 c. shredded mozzarella, divided; 1/3 c. sliced pepperoni (about 2 oz.)