This potato, leek, and broth base is a wonderful building block for an astounding number of flavorful, velvety soups. The recipe for fennel soup follows, but in place of the fennel, try adding a cup of chopped carrots, or corn off the cob, or chopped broccoli, or diced turnips. I could go on! The method is the same: First, you soften the vegetables, then add the broth and just a bit of cream, simmer, and purée. If you omit all the other vegetables, this becomes the perfect potato and leek soup; serve it chilled and you have a great vichyssoise.