Slice squash in half and remove seeds. Place halves in a dish, flesh side down. Cover with paper towel. Microwave on high for 8 to 12 minutes, or until tender. Let stand 5 minutes, then scrape out strands with a fork.
Heat oil in a large skillet over medium heat. Add onion and cook until soft, about 4 minutes. Add garlic and bell pepper; cook 2 minutes.
Remove veggies and place ground beef in skillet. Cook beef until browned, about 5 minutes. Return vegetables to pan with pasta sauce. Simmer mixture for 10 minutes.
Serve strands topped with meat sauce and garnish with Parmesan.