To make the sauce, heat oil in a large nonstick skillet over medium heat. Add onions; sauté 6 minutes until tender. Add garlic and cook until fragrant, about 30 seconds.
Stir in diced tomatoes, tomato paste, and crushed red pepper flakes and bring to a simmer. Reduce heat to low and simmer 10 minutes for flavors to blend.
Meanwhile, to cook squash, put squash in a microwave-safe plate or bowl. Cover and cook on High until strands of squash separate when flesh is scraped with a fork, about 8 to 10 minutes.
Scrape spaghetti squash from peel with a fork to form strands. Place into a large bowl; add sauce and fresh basil and toss to coat. Sprinkle servings with grated Parmesan.
Photo, recipe created and nutritional analysis completed by Terry Grieco Kenny for DietsInReview.com