Cullen skink with wholemeal soda bread

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