Carefully clean the leeks. You will need to cut off the outer, tough layers, and take the next few layers apart to rinse away the dirt and sand. I use any of the green part that is tender and all of the white part. After they are clean and trimmed, cut them across the grain into quarter inch sections. Dice the onion celery and potatoes.
Now sauté the leeks, onion and celery in a medium stock pot or extra large sauce pan. Cut the heat after the vegetables begin to wilt and put on the lid. After about a half hour, add broth. If you are using concentrated canned broth, don’t dilute it too much, you want as strong a flavor as possible.
Add the diced potatoes and simmer until ingredients are tender–30 to 45 minutes. Thicken the soup with instant mashed potatoes, adding a little at a time until it is to your liking. Taste and season. Turn off the heat and add either half and half or heavy cream, the amount is up to you.
Taste again for salt.
Use white part of leeks only, puree and chill. Season with white pepper. Garnish with chives.