Heat a large Dutch oven or soup pot over medium heat and melt the butter. Add chopped onions and cook until they start to brown.

Stir in minced garlic, Italian seasoning, salt, black pepper, and crushed red pepper flakes. Cook for about a minute until fragrant.

Pour in the chicken broth and mix well. Then add the whole chicken breasts and sun-dried tomatoes (if using).

Bring the soup to a gentle simmer, cover, and reduce the heat to medium-low. Allow it to cook for 12-15 minutes or until the chicken is fully cooked (internal temperature of at least 165°F), stirring occasionally.

While the soup simmers, cook the pasta separately in boiling salted water until al dente according to package instructions.

Once the chicken is cooked, remove it from the soup and shred it using two forks.

In a small bowl, whisk together the half and half (or cream) with corn starch until smooth.

Stir the shredded chicken, cooked pasta, cream mixture, and chopped spinach into the pot. Allow the soup to heat through, stirring occasionally, until it reaches your desired consistency.

Serve hot, garnished with a dollop of ricotta cheese, grated Parmesan, or shredded mozzarella, if desired.