Kidneys go 'off very quickly and must be eaten the day they are bought. Frozen kidneys may be used, but must be defrosted completely before using.
Serve with plain boiled white rice. This is a rich dish, which is best served with a simple green salad.
1. Heat the oil in a large frying-pan with a lid, add the onion and garlic, if using, and fry over moderate heat for 1 - 2 minutes. Add the kidneys to the pan and fry for a further 2 - 3 minutes, stirring all the time.
2. Stir the stock, vinegar, mustard, cayenne pepper and tarragon into the pan, with the grated orange zest and juice. Add the mushrooms, stir
well and season to taste with salt and pepper.
3. Bring to the boil, then lower the heat, cover the pan and simmer for 15 minutes until the kidneys are tender.
4. Transfer to a warmed serving dish, sprinkle with parsley and garnish with orange slices. Serve at once, piping hot.