Preheat oven to 400°F (200°C). Brush cut sides of squash with oil. Bake cut sides down on parchment paper-lined baking sheet 45 to 50 min., or until flesh is tender when pierced with a fork.
Meanwhile, arrange meatballs in lightly greased baking dish. Bake alongside squash, 10 to12 min., or until browned. Stir in jarred sauce. Cover dish with foil and continue to bake another 15 to 20 min., or until meatballs are cooked through.
Using a fork, gently scrape cooked strands from squash. Transfer to large platter. Serve with meatballs and sauce, garnished with parsley.
Serve with grated Parmesan cheese and pinch of hot pepper flakes.