Bring 3/8 litres of water, vinegar, rock candy, cinnamon, mustard seeds and cloves to the boil and let it boil for about 5 minutes. In the meantime clean, wash and drain the mirabelles. Add the mirabelles to the boiling stock and let them cool down.
Remove the cooled down mirabelle plums from the stock and pour into the glasses. Bring the stock to the boil again. Stir in the preserving aid and pour over the mirabelles so that they are covered. Close the jars immediately. Store in a cool and dark place.