I've made this soup since I got the cookbook, Julia Child's Kitchen, 30 years ago. This is super easy and delicious. If you don't have leeks, you can use any combination of onions, green onions, shallots, etc. You can also add leftover, cooked vegetables to vary it.
3 tbl butter
3 cups chopped leeks (white part only), or onions
3 tbl flour
4-6 cups water
1 tbl salt
Pepper to taste
1 cup or so of tender green part of leeks, chopped (optional)
4 cups (1 1/2 lbs) potatoes, peeled and chopped roughly
1 cup milk
1/3-1/2 cup heavy cream or sour cream and 2-3 tbl butter (I don't usually add this, but it does make it delicious)
2-3 tbl minced fresh parsley or chives
Melt butter in a 3-4 qt pot. Stir in the leeks/onions, cover pan and cook on low for 5 minutes without browning. Blend in flour and stir over moderate heat for 2 minutes without browning. Remove from heat and add 1 cup water, slowly, stirring. Return to heat and add remaining water, stirring. Add salt and pepper, optional leek greens, and the potatoes. Bring to a boil, reduce heat, and simmer partially covered for about 40 minutes.
Mash with a potato masher for a chunky soup or blend with stick blender for a smoother texture.
To serve, add milk and as much of the cream and butter as you'd like. Don't allow it to come to a boil again if you're using the milk. It may curdle. Taste and correct seasonings. Serve and garnish with chives or parsley.