Onions peel and chop finely. Peel, quarter, core and dice quinces. Caramelise the sugar in a large pot until golden brown.
Add onions, quinces and mustard seeds. Deglaze with wine and vinegar. Bring to the boil and simmer open for about 40 minutes. Clean the chilli, cut lengthwise, remove seeds, wash and finely dice. Stir into the relish and simmer for about 5 minutes.
Season to taste with salt. Pour into clean twist-off glasses. Close the jars and put them on the lid for about 5 minutes. Turn them over and let them cool down.