Start by heating up a large soup pot over medium heat and then adding the butter. Once the butter melts, add the celery, carrots and onions and cook until the vegetables soften up. This should take about 10 minutes.
Add the flour and cook for a couple of minutes being sure to keep stiring until the flour is incorporated nicely with the vegetables.
Add the chicken stock and bring to a boil but this time use a whisk to keep the liquids moving in the pot.
Make a sachet, a small bag of cheesecloth, with the bay leaf and fresh thyme and tie it closed with a piece of kitchen twine. You can add other herbs and spices to add additional flavors to the soup. Place the sachet into the pot.
Lower the heat to a simmer and cook for 15 minutes. Add the chicken and continue cooking long enough to heat up the cooked chicken.
Whisk in the heavy cream, taste and adjust seasoning with salt and pepper.
Before serving, remove the sachet. Nobody wants to find a sachet in their soup.
If you have some parsley, you may want to use it as a garnish to make the soup look even better. Homemade croutons are good too.