01 - Preheat the oven to 400°F. Line a baking sheet with parchment paper.
02 - Brush the cut sides of 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 squash bakes, season chicken breasts with olive oil, garlic powder, Italian seasoning, salt, and pepper.
04 - Heat a skillet over medium-high heat. Cook chicken 6-7 minutes per side 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 with squash strands. 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.