1. Preheat the oven to 180C/350F/gas mark 4. Put the fish fillets into a lightly buttered ovenproof dish and season with salt, pepper and lemon juice. Cover with foil and bake in the oven for about 20 minutes, until just cooked.
2. Drain off any liquid and quickly flake the fish, discarding the skin. Place in a shallow gratin dish.
3. Melt the butter in a saucepan and stir in the flour. Stir for a minute, then draw off the heat and gradually mix in the milk, a little at a time, followed by the cream. Bring to the boil and simmer for 510 minutes until thick, with no trace of raw flour taste.
4. Draw off the heat again and stir in the shrimps or prawns, then the egg yolks, salt and pepper. Pour over the cooked fish and sprinkle with Parmesan. Place under a preheated grill until nicely browned.
If youve cooked the fish in advance, or are using leftovers from another meal, prepare as above but finish the gratin in the oven at 190C/375F/gas mark 5, baking until nicely browned about 2025 minutes.