Step 1: Preheat your oven to 400°F (200°C). Carefully cut the spaghetti squash in half lengthwise. Scoop out the seeds and stringy bits. Drizzle the cut sides with a little olive oil, season with salt and pepper.
Step 2: Place the squash cut-side down on a baking sheet. Roast for 40-50 minutes, or until tender enough to easily shred with a fork. Once cooled slightly, use a fork to scrape out the strands, placing them in a large mixing bowl.
Step 3: While the squash is roasting, prepare your chicken if not already cooked. If using raw chicken breasts, season and bake at 400°F (200°C) for 20-25 minutes, or until cooked through. Shred or dice once cooled.
Step 4: Prepare the Alfredo sauce: In a large saucepan over medium heat, melt the butter. Add the minced garlic and cook for 1 minute until fragrant. Pour in the heavy cream and chicken broth, bringing it to a gentle simmer. Cook for 5-7 minutes, stirring occasionally, until slightly thickened.
Step 5: Remove the saucepan from the heat. Stir in 0.75 cup of grated Parmesan cheese, salt, pepper, and an optional pinch of nutmeg. Continue stirring until the cheese is melted and the sauce is smooth and creamy. Taste and adjust seasonings as needed.
Step 6: To the large mixing bowl with the spaghetti squash, add the cooked chicken and the creamy Alfredo sauce. Gently toss to combine, ensuring all strands are coated. Pour the mixture into a 9x13 inch baking dish.
Step 7: Sprinkle the top evenly with the shredded mozzarella cheese and the remaining 0.25 cup of Parmesan cheese.
Step 8: Bake the chicken alfredo spaghetti squash casserole for 20-25 minutes, or until the cheese is melted, bubbly, and lightly golden brown. If desired, you can broil for the last 2-3 minutes for an extra crispy topping (watch carefully!).
Step 9: Remove from the oven, let it rest for 5-10 minutes, then garnish with fresh chopped parsley before serving hot. Enjoy your healthier chicken alfredo spaghetti squash casserole!