Preheat the oven to 400°F (200°C). Place the halved spaghetti squash cut-side down in a baking dish with a little water. Bake for 30-40 minutes until tender.

In a skillet over medium heat, melt the butter. Add minced garlic and sauté for 1 minute. Add the cooked chicken and spinach, cooking until the spinach wilts, about 3-5 minutes.

Reduce heat to low and add heavy cream, stirring to combine. Gradually add parmesan cheese, salt, and pepper, stirring until the sauce thickens, about 5 minutes.

Once the spaghetti squash is done, use a fork to scrape the flesh into strands. Combine the squash with the chicken and spinach mixture, then pour the Alfredo sauce over it. Toss to combine and serve warm.