Classic fish pie with peas

Name: Classic fish pie with peas
Prep time: 30 min
Cook time: 2 h
Total time: 2 h 30 min
Cuisine: British
Ingredients: 8 medium baking potatoes; 1 tbsp vegetable oil; handful rock salt; 1 onion, thinly sliced; ½ tsp black peppercorns; 2 bay leaves; 500g/1lb 2oz boneless, skinless smoked haddock; 400g/14oz haddock, bones and skin removed; 400g/14oz cod, bones and skin removed; 200g/7oz butter; 3 heaped tablespoons plain flour; 475ml/17fl oz milk; 1 lemon, zest and juice; small bunch flatleaf parsley, leaves roughly chopped; 250g/9oz cooked peeled prawns; grated cheese, for topping; 200g/7oz peas; sea salt and freshly ground black pepper