Preheat the oven to 400°F.
Pour the Alfredo sauce into a 9x13-inch baking dish.
Fill the empty Alfredo jar with water, shake well, and pour into the baking dish. Stir in the chopped sun-dried tomatoes. Season with garlic powder, Italian seasoning, red pepper flakes and salt, if desired.
Add the uncooked pasta and stir until evenly coated. Cover tightly with foil and bake for 35–40 minutes, or until the pasta is tender.
Remove from the oven and stir in the chopped chicken and spinach. Let sit for 2–3 minutes until the spinach wilts, then serve warm.