Kidneys with mustard sauce

INGREDIENTS
4 lamb’s kidneys
30g (1oz) butter
1 tbsp olive oil
1 large onion, finely chopped
2 garlic cloves, crushed
8 chestnut mushrooms, sliced
3 tbsp vermouth or red wine
4 tbsp vegetable stock
2 tsp Dijon mustard
salt and freshly ground black pepper

METHOD
Peel away any skin from the kidneys and cut out the core and membranes. Soak for 5–10 minutes in a little milk or water, then drain, and pat dry on kitchen paper.
Heat the butter and oil in a frying pan, add the onion, and fry over a medium heat, stirring frequently, for 2–3 minutes. Increase the heat, add the kidneys and garlic, and fry for 2–3 minutes, stirring.
Add the mushrooms, fry for a further 2–3 minutes, then add the vermouth or red wine and the stock, and allow to bubble for 1 minute.
Reduce the heat, cover, and cook for 4 minutes, or until the kidneys are tender and cooked through. Stir in the mustard and season to taste with salt and pepper.