Pat chicken cubes dry and season with 1/2 tsp salt and 1/4 tsp black pepper. Heat 1 tbsp olive oil in a large pot or Dutch oven over medium-high heat. Add chicken and cook for 5-7 minutes, stirring occasionally, until browned on all sides and cooked through. Remove chicken from the pot and set aside.

Reduce heat to medium. Add 2 tbsp unsalted butter to the same pot. Once melted, add the diced yellow onion and cook for 5-7 minutes, until softened and translucent. Add the minced garlic and cook for another 1 minute until fragrant.

Sprinkle 1/4 cup all-purpose flour over the onions and garlic. Whisk continuously for 1-2 minutes to create a roux, allowing the flour to cook out.

Gradually whisk in the 6 cups of chicken broth, ensuring no lumps. Bring the mixture to a gentle simmer, stirring occasionally, and cook for 5 minutes, allowing it to thicken slightly.

Stir in the chopped sun-dried tomatoes, 2 tsp Italian seasoning, and 1/4 tsp red pepper flakes (if using). Continue to simmer for another 5 minutes, allowing the flavors to meld.

Reduce heat to low. Stir in the 1 cup heavy cream and 1/2 cup freshly grated Parmesan cheese until melted and fully incorporated. Do not bring to a rolling boil after adding cream and cheese.

Return the cooked chicken to the pot. Add the 5 oz baby spinach, pushing it down into the soup. Cook for 2-3 minutes, until the spinach has wilted.

Taste the soup and adjust seasoning with additional salt and black pepper as needed.

Ladle the creamy marry me chicken soup into bowls. Garnish with fresh chopped basil or parsley and extra Parmesan cheese, if desired. Serve hot and enjoy!