Bring a large pot of salted water to boil. Cook the pasta in the water until pasta is al dente, according to package directions. Gather the rest of your ingredients (even the spices) and make sure they are measured and on hand. Once the pasta has finished cooking, reserve about 1 cup of the pasta water. Drain the pasta and set aside.
Meanwhile, reserve 2 tablespoons of oil from the jar of sun-dried tomatoes, then drain away the remaining oil. Pat the tomatoes dry, then roughly chop and set aside.
Heat a large skillet over medium-high heat. Add the olive oil. Once it is hot, add the chicken pieces. Sprinkle with ¼ teaspoon salt and ¼ teaspoon pepper. Saute until the chicken is golden on the outside and fully cooked through on the inside, about 5 minutes. Remove to a plate and set aside.
To the skillet, add the reserved sun-dried tomato oil and the butter. Once the butter melts, add the sun-dried tomatoes and remaining ½ teaspoon salt. Sprinkle the flour over the top, then cook and stir for 1 minute, until the white bits of flour disappear. The pan will seem dry.
Slowly pour in the milk, stirring as you go. Bring to a simmer. Stir in the Italian seasoning, garlic powder, and remaining ¼ teaspoon black pepper. Let simmer until very slightly thickened, about 2 minutes.
Stir in the cheese until smooth. Turn off the heat and stir in the drained pasta and reserved chicken. Add the spinach a few handfuls at a time, until the spinach wilts and is fully incorporated. If the sauce is too thick at any point, splash in a little bit of the reserved pasta water as needed. It should be rich, creamy, and nicely coat the noodles. Enjoy hot, with a sprinkle of fresh basil or parsley as desired.
