Sauté the leek lightly in 1 Tbsp oil for 5-7 minutes.
In a stock pot add chicken stock, salt, celery, potatoes, and the sautéed leeks, and simmer 20-25 minutes.
With a slotted spoon, scoop out the potatoes and leeks to a small bowl, and using a potato masher, mash to a purée and then return to the stock pot.
Stir in the milk, and remaining oil or butter, reheating one minute if needed. Season to taste with salt and pepper. Garnish with green onion and nutmeg.