Cook meatballs according to package instructions.
While they are cooking, heat butter in medium saucepan over medium-high heat. Sauté onion until tender. Add garlic, Italian seasoning, black pepper and salt; sauté for 1 minute.
Add tomato sauce and paste; simmer for 10 minutes.
Add the cooked meatballs; simmer for another 5 minutes.
Slice hoagie rolls down the center and brush with melted butter. Place under broiler until toasted.
Return the subs to broiler until cheese is melted and browned.