First chop all the vegetables. If you have a food processor, you will get it done in no time!
Heat the butter and add the garlic, onion and leek and sweat for a minute. Followed by the rest of the vegetables, bay leaf and thyme. Stir and leave to sweat for a few minutes on medium heat.
Add hot chicken stock and cook for about 25-30 min until all vegetables are soft.
Add half of cooked chicken followed by milk and stir. You can take off the heat now and let it cool down. Spoon out the bay leaf and thyme before you blend it. Don't worry if the milk splits, it doesn't affect the taste and when you blend it, it won't matter.
Add remaining chicken pieces and season to taste. Serve with little bit of cream or yogurt and sprinkle with fresh herbs. Parsley works best in my opinion.
If you want richer soup add cream instead of milk after you blend everything.