Sauté the Vegetables: Melt the butter in a large stockpot or Dutch oven over a medium heat. Add the chopped onion, carrots, and celery. Cook gently for 8-10 minutes, stirring occasionally, until the vegetables have softened and the onion is translucent. Don't let them brown.

Create the Roux: Sprinkle the plain flour over the softened vegetables. Stir continuously for 2 minutes. This step cooks out the raw flour taste and is crucial for thickening the soup. I find that using a whisk at this stage helps prevent any lumps from forming.

Build the Broth: Gradually pour in the chicken stock, about 200ml at a time, stirring or whisking constantly after each addition to ensure the flour mixture dissolves smoothly. Once all the stock is incorporated, the mixture should be smooth.

Poach the Chicken: Add the whole chicken breasts, bay leaf, and dried thyme to the pot. Bring the liquid to a gentle simmer, then reduce the heat to low, cover with a lid, and let it cook for 15-20 minutes, or until the chicken is cooked through and no longer pink inside.

Shred the Chicken: Carefully remove the cooked chicken breasts from the pot and place them on a cutting board. Using two forks, shred the chicken into bite-sized pieces. Set aside. Remove and discard the bay leaf from the soup.

Blend for Smoothness (Optional): For an extra smooth, velvety consistency, you can use an immersion blender to partially or fully blend the soup base. I like to pulse it just a few times to break down some of the vegetables while still leaving some texture.

Finish the Soup: Return the shredded chicken to the pot. Stir in the double cream and the chopped fresh parsley. Gently heat the soup through for another 2-3 minutes, but do not let it boil, as this can cause the cream to split.

Season and Serve: Taste the soup and season generously with salt and freshly ground black pepper. Ladle into warm bowls and garnish with a little extra parsley if you wish.