This soup is simple but so satisfying. On a cold winter night the bright color and sweet, earthy flavor of the beets are the perfect way to warm your stomach. Put the beets and fennel in a small soup pot with just enough water to cover. Season with salt and pepper and simmer until the beets are completely tender. Use a blender to puree the soup in batches until completely smooth. Or you can puree half of it, if you like the variation in texture. Serve the soup with a dollop of sour cream or yogurt if desired, a squeeze of lemon and a few fennel fronds.