Preheat the oven to 150ºC / 300ºF / Gas Mark 2. Heat half the oil and 25g / 1 oz of the butter
in a flame proof casserole. When foaming, add the onions. Cover and cook over a low heat for 20
minutes until softened and beginning to collapse. Add the sugar and season with salt and pepper.
Stir in the thyme. Cover the casserole and transfer to the preheated oven. Cook for a further 30-45
minutes until softened completely, but not browned. Remove from the oven and stir in the balsamic
Meanwhile, boil the potatoes in boiling salted water for 15-18 minutes until tender. Drain well,
then return to the pan. Place over a low heat to dry completely, remove from the heat and stir in
50g / 2 oz of the butter, the milk, mustard and salt and pepper to taste. Mash thoroughly until
creamy and keep warm.
Heat a large frying pan and add the remaining butter and oil. When it is foaming, add the
mustard and sage leaves and stir for a few seconds, then add the liver. Cook over a high heat for
1-2 minutes on each side. It should remain slightly pink: do not overcook. Remove the liver from
the pan. Add the lemon juice to the pan and swirl around to deglaze.
To serve, place a large spoonful of the mashed potato on each plate. Top with some of the
melting onions, the liver and finally the pan juices.