For the marinade, heat vinegar, sugar and salt, stirring frequently until the sugar is completely dissolved. In the meantime, wash and peel the cucumber and cut in half lengthwise. Remove seeds with a spoon.
Cut the cucumber into 1 cm wide pieces. Peel onions and cut into rings. Peel, wash and chop horseradish. Wash coriander leaves. Layer ingredients with mustard seeds, coriander seeds and pimento seeds in a container.
Sprinkle with the hot marinade so that everything is covered. Close container well and store protected from light for 3-4 days until consumption.