You can use lamb, veal or ox kidneys for this dish, though smaller kidneys like lamb will obviously take a fraction of the time to cook. Over the years I've discovered that you can get very good results from ox kidneys and liver by actually cooking them slightly pink, in the same way as you would more expensive calves' offal and at a fraction of the cost.
Serve this dish with nice buttery mash or polenta or some greens.
2 large veal or ox kidneys, weighing about 700-800g each with the fat on, or about 600g cleaned of any fat
Vegetable oil for frying
Salt and freshly ground black pepper
For the mustard sauce
3 medium shallots, peeled and finely chopped
A good knob of butter
2tsp Dijon mustard
2tsp wholegrain mustard
50ml dry sherry
300ml double cream
1tbsp tarragon, chopped
Preheat the oven to 200C/gas mark 6. Season the kidneys and cook them in a little vegetable oil in an ovenproof frying pan for 4-5 minutes on a medium heat, turning so they colour evenly. Transfer to the oven for 10 minutes, keeping them pink. Remove from the oven and leave to rest for 10 minutes while you make the sauce.
To make the sauce, gently cook the shallots in the butter for 2-3 minutes, stir in the two mustards and sherry and simmer for a minute. Add the cream, bring to the boil and simmer until the sauce has reduced by half and thickened; season.
Halve the kidneys lengthways, then cut each half into thin slices, reserving any blood to add to the sauce. Add the kidneys to the sauce with the tarragon and return to the heat for a minute to heat the kidneys through.