Boil the potatoes in salted water for 15 minutes, or until tender. Drain and place back into the saucepan over the heat to remove excess moisture. Mash the potatoes.
Heat 150g/5½oz of the butter and the milk in a saucepan until the butter is melted. Add to the mash and beat well until soft and creamy. Season with salt and pepper.
For the onion rings, put the yeast and beer into a bowl and whisk together. Add the flour and whisk to a smooth batter. Set aside for 5 minutes.
Fill a deep-fat fryer or a large, deep, heavy-bottomed pan two-thirds full with vegetable oil. Heat the oil to 180C, or until a breadcrumb sizzles and turns golden-brown when dropped into it. (Caution: Hot oil can be dangerous. Do not leave unattended.)
Dip the onion rings in the batter then lower them carefully into the hot oil (you may need to do this in batches). When the onion rings are golden-brown, remove them using a slotted spoon and season with salt and pepper. Drain on kitchen roll.
Add the capers to the deep fat fryer and cook for 1-2 minutes. Remove and drain on kitchen paper.
For the gravy, heat the butter in a frying pan over a medium heat. When the butter is foaming add the shallots and fry for 2-3 minutes, or until just softened.
Add the thyme and red wine, stir well and bring the mixture to the boil. Reduce the heat to a simmer and then simmer until the volume of liquid has reduced by one third.
Add the jus and return the mixture to the boil. Reduce the heat slightly and simmer gently for 15-20 minutes, or until the liquid has thickened. Add the sherry vinegar, chervil and season with salt and pepper. Keep warm until ready to serve.
For the liver, season the plain flour with salt and pepper. Place the slices of calf's liver in the flour and shake off any excess.
Heat the remaining butter in frying pan over a medium heat. When the butter is foaming, add the liver and fry for 1-2 minutes on each side, or until golden-brown on both sides.
Place the mash onto serving plates and liver alongside. Pour over the gravy and top with the onion rings and capers.
