750g/1lb 10oz floury potatoes (Maris Piper or King Edward), peeled and cut into 2cm/¾in cubes
1 onion, finely chopped
2 tbsp chopped parsley
salt and freshly ground pepper
250g/9oz plain white flour
1 tsp bicarbonate of soda
1½ tsp table salt
250g/9oz wholemeal flour
50g/1¾oz butter, plus extra to serve
425ml/¾ pint buttermilk
1.2 litres/2 pints full-fat milk
450g/1lb undyed smoked haddock loin, skin and any bones removed