Peel and finely dice the onions. Wash, stone and dice the apricots. Weigh a total of 450 g onion-apricot mixture and mix with apple cider vinegar and mustard seeds in a wide saucepan. Gelling cane sugar (e.g.
of diamond) and bring to the boil while stirring. Simmer over high heat while stirring for about 6 minutes until bubbly. Pour immediately into prepared glasses.