Peel the cucumber and cut or grate into very fine cubes. Salt lightly and put in the refrigerator.
Put the garlic with oil, walnuts and yoghurt in the blender and beat thoroughly. Mix in the cream, season with salt and pepper.
Mix the cucumber cubes and yoghurt mixture together, season again to taste. Sprinkle on the dill and some retained walnuts, halved, and serve ice-cold.