Yup, that’s vichyssoise. I made some last week, so you’re in luck!
Take equal parts potatoes and leeks - I used about a pound of each. Slice the potatoes thin, and wash & slice the leeks into about 1/2 pieces. Throw 'em into a pot, and add chicken stock (homemade is best, but the Swanson stuff in the boxes works well, too. Get the low salt or organic version if you can) to almost cover. Put a top on the pot, and cook at a low boil until the pototoes are soft. Shouldn’t take more than ten minutes or so, depending on how thinly you sliced the taters.
Allow it to cool. Once it’s reasonably cool, put the whole mess in a blender. Depending on the size of your blender, you may have to do half at a time. Now, BE CAREFUL. If the mixture is at all warm, when you turn the blender on, you risk the steam blowing the top off of the blender. I cover the lid with a towel (well, I did, before I got my vita-mix, the king of blenders!), hold it firmly, and turn the blender on low speed. If all is well, keep turning the blender up and blend until it’s nice and smooth.
Once it’s blended, stir in about 1/2 cup heavy cream, and salt to taste. Serve cold, with chopped chives (and sour cream zigzags, if desired).