Prep and roast the squash: Heat the oven to 400°F (200°C). Halve the spaghetti squash lengthwise and scoop out the seeds. Rub the cut sides with a little olive oil and season with salt and pepper.
Place cut-side down on a parchment-lined sheet and roast 35–45 minutes, until tender and strands pull easily with a fork.
Brown the turkey: While the squash roasts, warm 1 tablespoon oil in a large skillet over medium-high heat. Add ground turkey, 1 teaspoon salt, and a few grinds of pepper. Cook, breaking it up, until browned and cooked through, about 6–8 minutes.
Transfer to a plate and keep warm.
Start the Alfredo base: In the same skillet, reduce heat to medium. Add butter. When melted, add minced garlic and cook 30–60 seconds until fragrant, not browned.
Make a quick roux: Sprinkle in flour and stir for 30 seconds to coat and cook off the raw taste.
It will look like a paste.
Add liquids: Slowly whisk in the chicken broth until smooth, then stir in heavy cream. Simmer gently 2–3 minutes until slightly thickened.
Finish the sauce: Stir in Parmesan a handful at a time until melted and smooth. Season with a pinch of nutmeg, more salt and pepper to taste, and red pepper flakes if you like a little kick.
Combine with turkey: Return the browned turkey to the pan and fold it into the Alfredo sauce.
Keep warm on low.
Shred the squash: When the squash is cool enough to handle, use a fork to scrape out long, spaghetti-like strands into a large bowl. Season with a little salt and pepper and a drizzle of olive oil if needed.
Toss and serve: Add the spaghetti squash to the skillet and gently toss to coat with the turkey Alfredo. Or, spoon the saucy turkey over squash strands in the hollowed shells for a fun “boat.” Top with extra Parmesan, chopped parsley, and a touch of lemon zest or a squeeze of lemon for brightness.
Optional add-ins: Fold in a couple of handfuls of baby spinach at the end, or sauté sliced mushrooms before browning the turkey.
Frozen peas are great too—stir them into the sauce for the last 2 minutes.