Skin and bone the fillets of fish and cut into 1 in (25 mm) squares. Peel and finely chop the onion. Melt the butter in a frying pan, add the chopped onion and cook gently for 5 minutes.

Stir in the flour, cook for a further minute, then gradually stir in the milk and bring to the boil.

Remove from the heat and add the bay leaf and the fish. Mix well, cover and leave aside for 20 minutes, and the sauce will start to cook the fish.

Add the oil and lasagne to a very large saucepan of salted boiling water, and cook according to the instructions on the packet. Drain thoroughly.

Stir the fish and remove the bay leaf, then stir in the chopped parsley. Drain and reserve the sweetcorn liquid if you are adding this to the sauce.

Stir in the sweetcorn and season to taste. Line a square or rectangular Ovenproof dish with a quarter of the sauce, cover with a third of the lasagne, add another quarter of the sauce and layer with more lasagne and continue, finishing with a layer of sauce.

Grate the Parmesan cheese and sprinkle evenly over the top, then place in the Oven and bake for 45 minutes, until the top is golden-brown and the lasagne is bubbling. Garnish with parsley and serve.