1. Cook the spaghetti according to package instructions until al dente. Drain and set aside, reserving some pasta water to adjust the sauce later if needed.
2. In a large pot, melt the butter over medium heat. Add the finely chopped onion and minced garlic, cooking until the onion becomes translucent and aromatic.
3. Sprinkle in the flour and stir continuously to form a roux, ensuring that there are no lumps.
4. Gradually pour in the milk, stirring constantly, and allow it to combine with the roux to form a thick and creamy base.
5. Introduce the sun-dried tomatoes to the sauce, stirring well. Add the Parmesan cheese and continue to mix until the cheese has melted into the sauce.
6. Season the sauce with salt and ground black pepper to taste. For extra flavor, you can add a pinch of red pepper flakes.
7. Gradually add the spinach to the sauce, stirring until it wilts and integrates well. If desired, this is the step to incorporate shredded chicken for more protein.
8. Add the cooked spaghetti into the pot with the sauce. Use tongs to toss the pasta and ensure it is evenly coated with the creamy mixture.
9. If the sauce appears too thick, add a splash of the reserved pasta water and stir until the desired consistency is achieved.
10. Serve the pasta in individual bowls or a large serving dish. Garnish with fresh basil, pine nuts, or additional Parmesan cheese if desired.
11. Enjoy this quick, flavorful, and satisfying meal, preferably warm, alongside your favorite complementary sides or beverages.