This is the simplest of simple soups. My mom made it often when I was at home. I don't even remember ever seeing a recipe. For our little bunch I use one leek. Craig calls this "the squeaky soup" (the leeks squeak a little between your teeth):
3 Tablespoons butter
2 Tablespoons olive oil
1-2 Leeks, dark green parts trimmed off, rinse very well, and slice crosswise
1-2 baking potatoes, peeled, cubed
2-4 cups water (or vegetable/chicken broth for a bit richer soup)--or half water half broth.
salt and pepper
Heat butter and oil over medium heat, add leeks and simmer for 5 minutes, add water or broth and potatoes and simmer until potatoes are tender. Smash several of the potatoes against the side of the pot to thicken the soup a bit then salt and pepper to taste.
Make it a meal and add these rolls to accompany: