Peel and chop the onion. Clean, peel and chop the beetroot. Heat butter. Fry onion and beetroot for about 5 minutes while stirring. Add vegetable stock and elderberry juice, bring to the boil and simmer for about 15 minutes.
Stir mascarpone and milk until smooth. Finely puree the soup with a hand blender. Season to taste with salt and pepper. Serve the soup and spread the mascarpone in streaks on the soup. Drizzle with oil and garnish with cress.
It goes well with baguette bread.