01 - Preheat the oven to 400°F. Line a baking sheet with parchment paper.
02 - Brush the cut sides of the spaghetti squash halves with olive oil. Season with salt and pepper. Place cut side down on the prepared baking sheet and roast for 35–40 minutes, until tender and easily pierced with a fork.
03 - While the squash bakes, season chicken breasts with olive oil, garlic powder, Italian seasoning, salt, and pepper.
04 - Heat a skillet over medium-high heat. Cook the chicken 6–7 minutes per side or until fully cooked (internal temperature 165°F). Remove to a plate, let rest 5 minutes, then dice or shred.
05 - In a medium saucepan, melt butter over medium heat. Add garlic and sauté 1 minute until fragrant. Pour in heavy cream and bring to a gentle simmer. Stir in Parmesan, mozzarella, and nutmeg. Cook, stirring, until cheeses melt and sauce thickens (about 3–4 minutes). Season with salt and pepper.
06 - When squash is cool enough to handle, use a fork to scrape the flesh into spaghetti-like strands, leaving about 1/4-inch shell intact. Transfer strands to a large bowl.
07 - Add chicken and Alfredo sauce to the bowl. Toss until well combined.
08 - Spoon the mixture back into the squash shells. Sprinkle evenly with mozzarella.
09 - Return boats to the oven (cut side up) and bake 10–12 minutes, until cheese is golden and bubbly.
10 - Garnish with fresh parsley if desired. Serve hot.