Preheat the oven to 350 degrees . Place the beets, potato, garlic, bay leaf and 4 cups water in a medium soup pot. Cover and bring to a boil; uncover and simmer over medium heat until the vegetables are tender when pierced with a fork, about 25 minutes.
Meanwhile, spread the pumpernickel cubes on a baking sheet and sprinkle with salt. Toast until crisp, about 15 minutes.
When the vegetables are cooked through, let the soup cool slightly and discard the bay leaf. Pour the soup into a blender, reserving about a cup of the liquid. Puree until smooth, adding enough of the reserved liquid to adjust the texture. Season with salt and return the soup to the pot to heat through. Serve in mugs topped with a spoonful of sour cream, a sprig of dill and some croutons.