1-First, prepare all ingredients. Season the chicken pieces with 1 teaspoon of the kosher salt and cut them into bite-sized pieces if needed.

2-Next, melt the butter in a large pot or Dutch oven over medium-high heat. Cook the chicken in batches until browned and cooked through, about 4 to 6 minutes per batch, then remove and set aside.

3-Then, add the diced onion, minced garlic, the remaining 1 teaspoon kosher salt, and black pepper to the pot. Cook over medium heat, stirring to soften the onion, for about 3 to 4 minutes.

4-After that, stir in the sun-dried tomatoes and tomato paste. Cook until the tomato paste darkens slightly, around 2 to 3 minutes.

5-Pour in the chicken broth, add the dried thyme and optional red pepper flakes, and bring to a boil.

6-Stir in the dried pasta shells, reduce heat to maintain a simmer, and cook until the pasta is al dente, about 10 to 12 minutes.

7-Finally, return the chicken and any juices to the pot, then add the baby spinach, heavy cream, and grated Parmesan cheese. Cook, stirring, until the spinach wilts and everything is warmed through, about 2 minutes. Adjust seasoning as needed and serve with more Parmesan if desired.