3 cups leeks cooked in 1 T. olive oil and 1 T. butter
4 cups chopped potatoes
5 cups water
a little salt
After the potatoes are soft, I put the soup through a food mill, and heat it again at a low temp.
Addendum May 4, 2009: Over this past winter, I've cut the amount of leeks down to maybe 1 1/2 cups. 3 cups seemed like too much. I even wonder if way back I used 3 leeks rather than 3 cups of leeks. Anyhow, we both like it better with less leeks. It still has great flavor but is not overpowering.
Addendum Two - March 31, 2010: I used just 1 T. butter instead of butter and olive oil. I also reduced the water to 4 cups to make a thicker soup.
The bread was toasted, and served with roasted garlic and roasted tomatoes.