Turn the stove to high heat now. Bring the water to a rolling boil. Simmer them until they are very soft.

Drain the potatoes well in your colander. Return them to the pot and mash them. Add a splash of milk for smoothness.

Chop all your fish into bite-sized cubes. Check for any small bones as you cut. Place the fish into your oven dish.

Scatter the frozen peas over the raw fish. Add the sliced leeks to the dish too. Spread them out in an even layer.

Mix the cornflour with a little cold milk. Whisk it until there are no lumps left. Pour the rest of the milk into a pan.

Heat the milk until it almost boils. Stir in the cornflour mixture very quickly. Keep stirring until the sauce gets thick.

Add the lemon juice and parsley to the sauce. Pour this hot liquid over the fish pieces. Ensure all the fish is covered well.

Spoon the mashed potato over the top. Use a fork to make small ridges. These will get crunchy in the oven.

Place the dish in a hot oven. Bake it at 200°C for thirty minutes. The top should be a golden color.