Peel and chop the onion and fresh beetroot. (Beetroot has a strong colouring - it is best to wear disposable gloves.) Heat butter in a large pot. Fry the diced onion and beetroot in it. Pour on 1/2 l water and elderberry juice, bring to the boil and stir in broth.
Cover and simmer for about 25 minutes.
Finely puree the soup with a hand blender. Season to taste with salt and pepper. Arrange and stir mascarpone in "streaks" into the soup. Drizzle with oil. Cut cress from the bed and sprinkle over it.
Ciabatta goes with it.