PREHEAT over to 400° F. Place spaghetti squash on a parchment paper lined baking sheet, cut side down. Bake 40 minutes.
MIX Meatballs: While squash roasts, in a medium mixing bowl, combine beef, minced onion, nutritional yeast, egg, 1 tsp basil, 1 tsp oregano, 1 tsp garlic powder, fennel seeds, sea salt and black pepper. Mix with hands.
FORM into 8 golf ball sized meatballs. Heat a large frying pan over medium heat and spray liberally with nonstick cooking spray. Place meatballs on pan and cook 3 minutes to brown. Turn meatballs and cook 3 minutes longer.
POUR in tomato sauce and diced tomatoes with meatballs. Stir in the rest of the garlic powder, oregano and basil. Reduce heat and cover. Simmer 20 minutes. Serve over 1 cup cooked spaghetti squash.
Alternate cooking method Instant Pot:
BROWN meatballs in instant pot. Add the tomato sauce and diced tomatoes with meatballs. Stir in the rest of the garlic powder, oregano and basil. Cook in high pressure for 10 minutes. Serve over 1 cup cooked spaghetti squash.