Preheat oven to 375°. Stab spaghetti squash with a fork all over to prevent bursting.
Place on a rimmed baking sheet and bake in the center of the oven for an hour, or until fully soft when stuck with a fork. Remove from oven and let cool.
When squash is cool enough to handle, cut in half and remove seeds. Gently scoop out squash noodles with a fork, keeping noodles intact, and put in a serving bowl.
Serve squash topped with tomato sauce and meatballs. Pass the parmesan cheese at the table.
Squash may be made a few hours ahead of time and reheated in a covered baking dish on 300° until warm.