Devilled kidneys

Name: Devilled kidneys
Prep time: 30 min
Cook time: 30 min
Total time: 1 h
Cuisine: British
Ingredients: 6 lambs’ kidneys, about 375g/13oz, skinned; 2 tbsp plain flour; 25g/1oz butter; 1 onion, thinly sliced; 1 tbsp tomato purée; 1 tbsp English mustard; 1–2 tbsp Worcestershire sauce; 4 thick slices crusty bread; butter, for spreading; small bunch flatleaf parsley, finely chopped (optional); salt and freshly ground pepper