Heat the olive oil in a large soup pot or Dutch oven over medium-high heat. Add the diced chicken, 1 teaspoon of Italian seasoning, salt, and pepper. Cook until browned on all sides, about 4–5 minutes.
In the same pot, add the onions, carrots, celery, sundried tomatoes, and garlic. Cook until the vegetables are translucent, about 3–4 minutes.
Sprinkle the flour over the mixture and stir well to combine, adding it gradually to avoid clumping. If using tomato paste, stir it in at this stage.
Gradually whisk in the chicken broth while stirring to ensure no lumps form. Scrape the bottom of the pot to deglaze any browned bits.
Bring the mixture to a rolling boil. Add the pasta, remaining 1 teaspoon of Italian seasoning, salt, and pepper. Cover the pot, lower the heat to a simmer, and cook for about 20 minutes, or until the chicken is cooked through and the pasta is al dente. (Alternatively, cook the pasta separately and add it at the end to prevent it from becoming mushy in leftovers.)
Stir in the heavy cream, spinach, and grated Parmesan cheese. Let the soup simmer for an additional 5 minutes.
Taste and adjust seasoning with additional salt and spices, if needed. Serve warm.
Notes
Broth Consistency: Start with 6 cups of broth for a thicker soup. Add more later if you prefer a thinner consistency.
Tomato Paste: The tomato paste is optional. It adds a slight tomato flavor, but you can omit it for a creamy, white-based soup.
Pasta Tips: To prevent mushy pasta, cook it separately and add it before serving. This is especially helpful when storing leftovers, as pasta can absorb liquid over time.
Using Rotisserie Chicken: For convenience, substitute rotisserie chicken. If using pre-cooked chicken, add it at the end and heat through.
Reheating: When reheating, add extra broth or water to thin the soup if needed.
Flour Substitutions: Flour thickens the soup, but you can omit it if desired. Cream cheese can be used as an alternative thickener.
Cream Alternatives: Swap heavy cream for half-and-half, milk, or full-fat coconut milk. These alternatives will change the texture slightly.
Slow Cooker Option: Add all ingredients except the pasta, cream, spinach, and Parmesan to the slow cooker. Cook on Low for 8 hours or High for 4 hours. Boil the pasta separately, then add it to the slow cooker along with the cream, spinach, and cheese. Stir and serve.