Add all of the ingredients, except 3/4 cup of the shredded mozzarella and the sliced pepperoni, to a 9x13" disposable aluminum tray.
1 lb ground sausage, 12 oz cream cheese, 2 3/4 cups shredded mozzarella, 1/4 cup shredded Parmesan cheese, 2 cups marinara sauce, 1 tbsp minced garlic, 1 tsp Italian seasoning, 1/4 tsp crushed red pepper flakes