In a deep saucepan melt the butter with sugar on a medium heat. Add the onion and cook for 20-30 min with the lid covered. The onion should become transparent and get a bit caramelised
Add the coriander, wine and vinegar. Season to taste
Continue cooking for 30 min on a low heat until the liquid is reduced
Transfer to the clean jars and keep in the fridge
I took sweet white onion, icing sugar and 1 tbsp of honey. For a long time could chose between white or red wine, so, took rose at the end, in which I soaked the raisins. I took less of coriander, as was afraid it would overbalance the foie gras.
Leave the confiture out for some time before using, as butter does get it hardened in the fridge.
I served the confiture on the foie gras mini-tarts.