Melt 20g butter in frying pan on medium-high heat. When foaming add the onions and fry for 15-20 mins stirring occasionally until very soft and golden. Set aside on plate.
Season livers add remaining butter and when it foams add livers to pan. Cook for 2-4 mins on each side until browned (a little pink on the inside is fine).
Add sherry (or wine) to liver allow to bubble for 30 secs, add cooked onions and stock, bring to the boil, reduce heat and cook for couple of minutes until stock has reduced slightly. Serve immediately with lots soft mustard mash.