Sweat the sliced onions in the butter over a low heat, add the salt and cook until tender.Add the sugar, 250 ml water, red wine and the vinegar. Increase the heat and cook until the liquid evaporates.
Taste the red onion doneness and add the rest of the water if necessary.
When the onion relish is cooked it will be sticky like jam.
Season to taste and transfer into sterilised jars, close the lid, and allow to cool.