School prawn and potato bake

Toss the peeled prawns in a bowl with half the lemon juice and a pinch of salt. Place the diced potato in a pot, cover with water and bring to the boil. Simmer until tender. Drain and mash the potatoes with 20 grams of the butter, two egg yolks and the nutmeg until smooth. Place 60 grams of the remaining butter in a pot and melt on a moderate heat. Once melted, add the flour and stir continually until it has turned a light, golden colour. Gradually add the boiling milk and keep stirring. Reduce to a simmer and cook, stirring constantly, for 15 minutes as it thickens. Divide this mixture into two equal parts. To the first add the school prawns (saving 24 prawns for the top) and the parsley; to the second add the remaining egg yolks and 20 grams butter. Season with salt and pepper and mix both well. Line four ovenproof ramekins, or one larger pie dish, with the potato puree, making sure the puree lines the bottom as well as the sides of the dishes. Distribute the mixture with the prawns evenly on the potato and then the last mixture on top. Press the saved prawns lightly into the top mixture and bake in a preheated 180C oven for 20-25 minutes until they are golden.