Cut the veal kidneys into small pieces respecting their natural divisions. Remove any unwanted sinew or fat. Spread the pieces of kidney out on a plate.

In a large and not-too-heavy frying pan, or a wok if you prefer, heat 2 tbsp butter and the olive oil until foaming. Season the kidneys with salt and pepper and throw into the pan. Sear briefly before tossing and turning in the butter and oil for a very short time until the kidneys have taken on a little color, but are not overcooked—they need to be rare, as they are going to be reheated. Turn the kidneys into a colander over a plate and leave to drain.

Return the juices to the pan, add the shallots, and cook gently for about 10 minutes or until golden brown and gooey, adding a little more butter if necessary.

Remove from the pan with a wooden spatula and add to the kidneys.

Wipe the pan clean with a paper towel and heat the remaining butter in it until golden brown, i.e., beurre noisette.

Throw in the sage leaves and fry until crisp, being careful not to burn them or the butter.

Turn off the heat, lift out the sage leaves, and place them on paper towels. Lightly salt them.

Reheat the butter and return the kidneys and shallots to the pan. Stir-fry for a few moments, and pour in the vinegar. Sizzle for a few seconds, then add the finely sliced spring onions. Spoon onto a heated serving dish, top with the sage leaves, and serve piping hot with mashed potatoes.