In a large saucepan over medium heat, add the olive oil and chopped onion. Saute until translucent. Add in the minced garlic and stir.
Pour in the crushed tomatoes, Italian seasoning, sugar, and salt & pepper. Stir together and bring to a simmer.
Add in the meatballs, turn heat to low, cover, and let heat through - about 5-8 minutes.
Once heated through, toast your sub rolls and pile on meatballs and cheese.
Let cheese melt and enjoy!