01 - Melt butter in a large pot over medium heat. Add chopped onion, celery, and carrots. Sauté for 4-5 minutes until vegetables begin to soften.
02 - Stir in minced garlic and cook for 1 minute until fragrant, being careful not to brown the garlic.
03 - Sprinkle flour over the vegetables and stir constantly for 2 minutes to cook out the raw flour taste and form a smooth roux base.
04 - Gradually whisk in chicken broth, ensuring no lumps form. Bring the mixture to a gentle simmer while stirring frequently.
05 - Add cooked chicken, dried thyme, salt, and black pepper. Simmer uncovered for 15 minutes until vegetables are completely tender.
06 - Reduce heat to low. Stir in whole milk and heavy cream. Heat gently for 5 minutes, stirring occasionally. Do not allow the soup to boil.
07 - Stir in fresh parsley and adjust seasoning to taste. Ladle into bowls and garnish with additional parsley if desired.