Bring half a stock pot of water or stock to a boil. (Eyeball the amount of water compared to the amount of vegetables you will be using, you want a couple of inches of water over the top of the honored ingredient.) Wash, roughly slice vegetables to a uniform size, and add them to the water. Leave to boil for at least thirty minutes, depending on the done-ness of the vegetables. You may have to skim some scum off the top of the soup if you are using parsnips.
When the vegetables are in the interval between thoroughly cooked and boiled to hell, salt the soup, and puree in a blender or with an immersion blender. I prefer to puree at a high speed for the maximum emulsion, which gives the creamiest mouthfeel. If you are making this soup in steps, it's important to heat the soup before it is blended. (Unless you are a lucky SOB with a Vitamix. Then many rules don't apply to you.) The only tricky thing to making this soup is getting the water levels right. Since some of the vitamins will leech from the veggies, you don't want to discard the water or stock post-boiling and lose those nutrients. At this step, if you wish, add a pat of butter. My guess is you'll be surprised you don't need it, because this velvety puree will hit all the right notes. Grind some fresh pepper over the top and serve plain or with a dollop of creme fraiche, yogurt, or croutons. Some like shredded cheese as well.