Calves' sweetbreads aren't a cheap cut, but they are delicious. You will probably need to order calves' sweetbreads in advance from your butcher or try Donald Russell (donaldrussell.com), who sells a great selection of offal and cuts.
800g calves' sweetbreads, soaked in cold water for a couple of hours
700-800g medium carrots, peeled
1tbsp chopped chervil or parsley
Salt and freshly ground black pepper
Preheat the oven to 200C/gas mark 6. Cut the carrots into 5cm lengths, then square them off, keeping the trimmings for the purée. Cut the lengths into cm batons, adding some to the trimmings so that you have two evenly-sized piles, one of batons and one of batons mixed with trimmings.
Put the pile that includes the trimmings into a pan, cover with water and add a teaspoon of salt, bring to the boil and simmer until the carrots are tender; then drain, reserving the cooking liquid, and blend the carrots in a food processor until smooth. Transfer to a saucepan, add a little butter; season to taste.
Cut the sweetbreads into 4 evenly sized chunks or, if they are odd shapes, 8 pieces. Heat a tablespoon or so of the vegetable oil in preferably an ovenproof frying pan, season the sweetbreads and fry them on a high heat on all sides until they are nicely coloured and transfer to the oven.
After 5-6 minutes, add half of the butter and baste the sweetbreads; return to the oven for another 10 minutes, turning them a couple of times during cooking. Meanwhile, cook the batons of carrots in the liquid from the trimmings for a couple of minutes or until they are cooked but still have a bit of a bite, then drain.
Remove the sweetbreads from the pan and keep warm. Drain the fat from the pan, add the Riesling and simmer until there is about a tablespoon left, then remove from the heat and whisk in the remaining butter before adding the batons of carrots and chervil.
To serve, reheat the carrot purée and spoon on to the centre of warmed plates, place the sweetbread on top and spoon the carrots and sauce over.