Sauté the onion, leek and celery in butter. Add the peeled cucumber pieces and steam. Dust with flour and cool a little. Add the hot vegetable broth and bring to a boil, stirring throughout. Simmer until vegetables are tender, then mash and pass through a sieve.
Bring to the boil again, stir in the cream and milk and season to taste. Refrigerate.
Before serving, sprinkle with diced cucumber, top with a dollop of yogurt and garnish with dill.
Tip: Use creamy natural yogurt for an even finer result!