Cut the chicken into bite-sized pieces and season with 1 teaspoon of kosher salt.

Melt the butter in a Dutch oven over medium-high heat. Cook the chicken in batches until browned and cooked through, about 4-6 minutes per batch. Transfer to a plate.

Add the diced onion, minced garlic, black pepper, and the remaining salt to the pot. Cook until the onion is soft and translucent, about 3-4 minutes.

Stir in the sun-dried tomatoes and tomato paste. Cook until the paste darkens slightly, about 2-3 minutes.

Add the chicken broth, thyme, and red pepper flakes. Bring to a boil.

Stir in the pasta and reduce heat to a simmer. Cook until the pasta is al dente, about 10-12 minutes.

Return the chicken to the pot, adding any juices. Stir in the spinach, heavy cream, and Parmesan cheese. Cook until the spinach wilts, about 2 minutes. Adjust seasoning if needed.