Balkan cold yoghurt soup with cucumber, walnuts and sumac

This Balkan cold soup is made with Greek yoghurt and kefir or ayran, diced cucumber, garlic, dill, olive oil and lemon juice. It is served topped with toasted walnuts, mint, dill and sumac. The dish resembles the Polish cold cucumber soup but is distinguished by the walnut and mint additions; it can be served alongside young boiled potatoes with dill.