lb. refrigerated cheese ravioli • olive oil, divided • onion, minced • lb. sweet or hot Italian sausage, removed from casing • garlic cloves, minced • red pepper flakes • oregano • (28 oz) tomato puree • (14.5 oz) diced tomatoes, drained • mozzarella cheese, shredded (1 1/3 c.) • Parmesan cheese, grated (1/2 c.)