Season the diced chicken with half of the salt and black pepper. Heat olive oil in a large Dutch oven or heavy-bottomed stockpot over medium-high heat. Add the chicken and cook for 5 to 6 minutes, stirring occasionally, until lightly browned and cooked through. Transfer the cooked chicken to a plate and set aside.

Lower heat to medium. Add the diced onion and sun-dried tomatoes to the remaining oil in the pot and sauté for 3 to 4 minutes until the onion turns translucent. Stir in the minced garlic, Italian seasoning, and red pepper flakes, cooking for about 1 minute until fragrant.

Sprinkle the all-purpose flour over the sautéed vegetables and stir constantly for 1 minute to cook out the raw flour flavor. Slowly pour in the chicken broth while whisking continuously, scraping up any browned bits stuck to the bottom of the pot.

Bring the liquid to a gentle boil, then reduce the heat to medium-low. Add the cheese tortellini and return the browned chicken along with its resting juices to the pot. Simmer uncovered for 4 to 5 minutes, or until the tortellini are tender and float to the surface.

Reduce the heat to low. Stir in the heavy cream, freshly grated Parmesan cheese, and fresh baby spinach. Simmer gently for 2 to 3 minutes, stirring constantly, until the cheese has melted smoothly into the soup and the spinach has fully wilted. Season with additional salt and black pepper to taste.

Ladle the hot soup into warm bowls and garnish with chopped fresh basil and extra grated Parmesan cheese. Serve immediately with crusty Italian bread.