Pre-heat oven 180°c.
Pour the milk into a saucepan with the bay leaf, pepper corns, garlic and dill. Bring to the boil and then reduce the heat.
Add the hake and simmer until the fish is flakey. Remove the fish and strain the milk, reserving it to make the sauce.
In a pan, heat the butter and gently cook the onion for 5 mins. Then, stir in the flour and cook while stirring for 2 minutes. Slowly add the milk and stir until you have a nice smooth sauce. Add the cheese and stir until it has melted, season with salt and pepper, and add mustard.
Flake the hake and stir it into the sauce, add the peas and prawns. Stir gently together, spoon into a baking tray and top with mashed potato. Bake for about 30 minutes or until golden and bubbling.
Serve with a mixed green salad.