Boil the egg hard and let it cool down in cold water. Wash the cucumber, cap both ends, do not peel, halve lengthways and cut across into 6 mm thick slices. Halve the onion lengthways, cap both ends, peel, quarter the halves lengthways and cut them crossways into approx. 2 mm thick slices. Wash the tomatoes, remove the stem, quarter lengthways, remove the green stem and cut in half crosswise.