1. Put the milk and butter in a pan and heat to boiling point, then reduce heat to a simmer.
2. Put the fish in the milk and poach over a low heat, turning once. It should take only a few minutes to cook through.
3. Remove the cooked fish, retaining the milk mixture and flake the fish, carefully removing all bones.
4. Place the flaked fish into a small oven proof dish with the frozen peas. Mix 2 tbsp of the cheese with the yogurt and spoon over fish mixture
5. Mash the potato, adding a little of the reserved milk and use to top the fish and cheese mixture. Sprinkle over the remaining cheese and bake at 200°C for 20 minutes.