Preheat oven to 400°F. Cut spaghetti squash in half, scoop out seeds, drizzle with olive oil, and roast face-down on a baking sheet for 35–40 minutes.
While squash roasts, heat a skillet over medium heat. Brown the Italian sausage, breaking it apart with a spoon. Add onion and garlic, cook until fragrant.
Pour in chicken broth and let it simmer for a few minutes. Stir in heavy cream and red pepper flakes if using.
Scrape roasted squash into strands with a fork and add them to the skillet. Toss everything together until well combined.
Season with salt, black pepper, and Parmesan. Garnish with fresh parsley before serving.