Coarsely chop the onion

Spray the slow cooker bowl with cooking spray

Pour the spaghetti sauce and tomatoes into the slow cooker bowl

Add the onion, minced garlic, and Italian seasoning

Stir until all ingredients are well mixed

Add frozen meatballs

Stir again, making sure meatballs are coated in sauce

Cover the slow cooker

Cook on low heat for 8 hours

Fill a large pot 3/4 full with water

Add oil to the water

Bring the water to a boil on the stovetop over high heat

Add the spaghetti to the boiling water

Cook for 7 minutes, stirring frequently

Save one cup of pasta water

Drain the spaghetti

Add the spaghetti to the slow cooker bowl

Add the reserved pasta water to the slow cooker bowl

Stir until all pasta is coated

Sprinkle with parmesan cheese

Serve