01 - Preheat oven to 400°F. Line a baking sheet with parchment paper.
02 - Halve each squash lengthwise and remove seeds. Drizzle insides with olive oil, season with salt and black pepper. Arrange cut sides down on the baking sheet and roast for 35 to 40 minutes, until tender.
03 - In a medium saucepan over medium heat, melt unsalted butter. Sauté minced garlic for 1 minute until aromatic.
04 - Pour in heavy cream and whole milk, stirring continuously. Bring to a gentle simmer, whisk in grated Parmesan cheese, ground nutmeg, salt, and black pepper. Cook until sauce is smooth and thickened. Remove from heat.
05 - In a large mixing bowl, add cooked chicken, chopped spinach, sliced mushrooms if using, and half of the Alfredo sauce. Mix gently until evenly coated.
06 - When squash is cool enough to handle, use a fork to shred the flesh into strands, keeping the shells intact.
07 - Add shredded squash strands to the chicken mixture, and toss gently to combine.
08 - Spoon mixture back into the hollowed squash shells. Drizzle the remaining Alfredo sauce on top and sprinkle with shredded mozzarella cheese.
09 - Return filled squash halves to the baking sheet and bake for 10 to 12 minutes, until cheese is melted and bubbling.
10 - Finish with fresh chopped parsley before presenting.