Pour the oil from the jar of sun-dried tomatoes into a cast iron pan and set the tomatoes aside; slice them if desired.
Season the cubed chicken with 1 teaspoon Italian seasoning, salt, paprika, red pepper flakes, and black pepper; toss to coat.
Heat the pan over medium heat and brown the chicken in the reserved oil until cooked through; sprinkle with 1/4 cup grated Parmesan, then remove the chicken from the pan and set aside.
Add butter to the pan, then sauté the diced onion and minced garlic over medium heat until the onion is tender, about 5 minutes.
Stir in the remaining 1 teaspoon Italian seasoning, then add the water, heavy whipping cream, and uncooked pasta to the pan.
Bring the mixture to a rapid boil over medium-high heat, stirring occasionally, and cook until the pasta is tender, about 8–12 minutes.
Reduce heat to low and stir in the spinach, cooked chicken, and sliced sun-dried tomatoes; cook until everything is heated through and the spinach wilts.
Remove from heat and top with shredded Parmesan before serving.
