1 onion, thinly sliced
small bunch flatleaf parsley, finely chopped (optional)
1 tbsp English mustard
2 tbsp plain flour
1–2 tbsp Worcestershire sauce
salt and freshly ground pepper
1 tbsp tomato purée
25g/1oz butter
butter, for spreading
6 lambs’ kidneys, about 375g/13oz, skinned
4 thick slices crusty bread