I like this soup in the fall when leeks are cheap. As in 69 cents each. When they are $2.69 each, I don't like them as much. Plus, they are cheap when potatoes are cheap also - 50 cents a lb - so a big win on the budget front.
To determine calories, you must of course base it on what you use. Your calorie count will vary...
Leeks (3 @54 calories each, 162 calories total)
Yukon Gold Potatoes, peeled (18 ounces @ 26 calories per ounce, 468 calories total)
Swanson or Progresso Chicken Broth, low sodium (8 cups, 15 cal per cup, 120 calories total). Or go with vegetable broth.
1 cup water (free)
salt and pepper to taste (free)
Thyme to taste (free)
Yields 8 cups, so 94 calories per cup with what I used.
Cut the dark green portions of the leeks off, slice in half the long way, then slice fine. Put in a big pot of water and swish around to get the dirt out of them. Drain.
Add to big pot and sweat (that is, cook at low to medium heat till they soften). You don't need oil, butter or anything else. They will release moisture as they cook down. Salt with about a teaspoon of salt and a teaspoon of pepper.
Peel and slice potatoes into 1/4 thick slices. I do it this way because they then cook evenly no matter how big the potato is - every slice cooks at the same speed.
When leeks have gotten soft (10 to 15 minutes), add potatoes, thyme, broth and water. Cook at medium until potatoes are soft
Put half at a time into a blender, blend till smooth - be careful and put a towel over the blender and don't fill it more than 1/2 way - to much is a good way of blowing the top off, burning yourself and painting the walls with the soup.
When pureed, serve. Good hot, good cold.