1. Clean the kidney, remove the fat and cut it in thin slices. Blanch it in boiling water during 30 seconds and strain it. Peel and chop the onion and a clove of garlic; clean, dry and chop the parsley.
2. Melt the lard, or heat the oil, in a big frying pan. Add 3 or 4 slices of kidney, 1 clove of garlic and the slice of white bread. When the kidney is fried, put it in a mortar, jointly with the garlic and the bread. Add a bit of water and crush until you obtain a homogeneous paste. Keep it until further use.
3. Melt the same lard, or heat the same oil, in the frying pan. Add the other kidney slices and the chopped onion and garlic, as well as a teaspoonful of chopped parsley. Toss it in oil during some minutes, always stirring, wash it down with the sherry, add salt and cook it over a gentle heat. After 5 minutes, add a glass of water and let it cook during 15 minutes more.
4. Then, add the paste from the mortar, stir smoothly, add salt if necessary and let the sauce thicken.