Melt the butter in a heavy saucepan. Add the leek and onion and cook over low heat until soft and translucent, 15 minutes.
Add the potatoes, beet, stock, and lemon juice. Bring to a boil, reduce the heat, and cover. Simmer until the potatoes and beet are tender, 40 minutes. Allow to cool slightly.
Process the soup in a blender or food processor until