Luxury Fish Pie With Cheesy Potato Rösti Topping

Name: Luxury Fish Pie With Cheesy Potato Rösti Topping
Ingredients: 600 g fresh cod fish fillets or 600 g halibut steaks; 250 g scallops, cut in half; 250 g tiger shrimp, peeled and devained (defrosted if frozen); 500 ml fish stock or 500 ml vegetable stock; 1 bay leaf; 50 g butter; 50 g plain flour; 2 tablespoons double cream; 5 -6 gherkins, drained and chopped; 2 tablespoons chopped fresh parsley; salt & freshly ground black pepper; For the rosti topping; 1 kg desiree potato, nicely cleaned; 50 g butter, melted; 50 g cheddar cheese, finely grated