Cook the chicken: Heat olive oil in a large pot over medium heat. Season the chicken with salt and pepper, then cook the chicken breasts or thighs until golden brown and cooked through, about 6-8 minutes per side. Remove the chicken from the pot and set aside to rest. Once cooled, shred the chicken into bite-sized pieces.

Sauté the aromatics: In the same pot, add the diced onion and cook for 3-4 minutes until softened. Add the minced garlic and cook for an additional 1 minute until fragrant.

Make the soup base: Add the chicken broth, heavy cream, chopped sun-dried tomatoes, Italian seasoning, and red pepper flakes (if using) to the pot. Stir to combine and bring to a simmer.

Thicken the soup (optional): If you want a thicker soup, mix the cornstarch with water and add it to the pot. Stir and let the soup simmer for an additional 2-3 minutes until thickened to your liking.

Add the chicken: Return the shredded chicken to the pot and stir it in. Let the soup simmer for 5-10 minutes to allow the flavors to meld together.

Finish with Parmesan: Stir in the grated Parmesan cheese until melted and smooth, adding extra seasoning if needed.

Serve and garnish: Ladle the soup into bowls and garnish with freshly chopped basil or parsley.