Chop the leak and saute in a bit of peanut oil. Chop onion coarsely and add to leek. Peel head of garlic and add garlic cloves to leek/onion mix. Cut up potatoes and add to mix.
Add 2 cups of chicken stock and 1 cup water and a handful of salt. Boil entire mixture for 25-30 minutes. Remove solids from stock and puree the solids. Pour pureed solids back into stock.
Add a bit of milk or cream (1/2 to 1 cup) to thicken.
Serve with croutons and garnish with chervil.