Put the oven on to gas mark 6 (200°C/400°F) so it can heat up. Peel and cut the potatoes into chunks. Add to a saucepan of boiling water and cook for 15-20 minutes until soft.

Meanwhile, prepare the leeks: cut off the base and the top part of the leaves, remove the outer layer, and cut the leeks in half along their length. Slice the leeks and wash thoroughly under running water. Cut the fish pieces into cubes and remove any bones.

Heat the oil in a frying pan over a medium heat. Fry the sliced leeks for about a minute, then add the cubes of fish and continue cooking for another 5 minutes. Stir in the peas and cook for another minute.

When the potatoes are cooked, drain off the water using a colander or sieve, and mash with a little milk and butter or margarine. Arrange the fish, leeks and peas in an ovenproof dish, pour the cheese sauce over the top. Cover the mixture evenly with the mashed potato, and bake in the oven for 30 minutes until the top is golden.