Preheat the oven to 375°F (190°C). Trim the ends of the spaghetti squash and slice it in half lengthwise. Use a spoon to scoop out and discard the seeds.
Drizzle 1–2 tablespoons oil per half over the cut sides of the squash and rub to coat; sprinkle with sea salt. Place cut-side down on a baking sheet and roast 40–55 minutes until strands reach desired tenderness.
Remove the squash from the oven and let cool until you can handle it safely, about 10–15 minutes.
While the squash roasts, heat 1–2 tablespoons oil in a large (about 12-inch) skillet over medium heat. Add the chopped onion and sauté until softened, about 5 minutes.
Add the chopped chicken and minced garlic to the skillet, spreading into an even layer; cook 5–6 minutes, stirring occasionally, until the chicken is cooked through and lightly golden.
Stir in the paprika, red pepper flakes (if using), coconut milk, and drained sun-dried tomatoes. Bring to a boil, then simmer, stirring occasionally, until the sauce thickens to a creamy alfredo-like consistency, about 10–15 minutes.
Add the baby spinach, cover, and cook 2–3 minutes until wilted; stir to combine with the sauce.
Use a fork to shred the roasted squash into strands and transfer them to the skillet. Toss gently to combine and heat through. Taste and adjust sea salt; add a squeeze of lemon juice if desired.
Serve in bowls and enjoy.