I've never heard of this combination before I found it in Mireille Guiliano's The French Women Don't Get Fat Cookbook, and I was a little suspicious. Generally, I think it's pretty great. And unlike most soups, I actually prefer this one cold. It's very refreshing!
4 tablespoons unsalted butter
1 lb carrots, chopped
zest of 1 orange
1 medium potato, peeled, chopped
4 cups vegetable stock
1/2 cup milk
If you don't keep vegetable stock on hand (like me), separate the green parts from the rest of the leek. Put the greens in a pot with water (definitely use at least 4 cups). Bring to a boil, then simmer, covered, for about 30-45 minutes. Discard greens and set aside stock.
In a large pot, melt the butter over medium-high heat. Add the leeks and sautee until softened, about 3-5 minutes.
Add carrots, orange zest, potato, and vegetable stock and bring to a boil. Cover, lower the heat, and simmer for 30 minutes, or until vegetables are tender.
Carefully transfer mixture, small portions at a time, to a blender and puree until smooth. Return to pot and add milk. Serve hot or cold, with pepper and a small spoonful of cheese (I prefer goat).