This makes a fantastic first course if you only have a few langoustines; however, if you have more, there is no reason why it could not be turned into a main course! When poaching the eggs, try doing it the restaurant way: set a deep pan of water to rapidly boil with a touch of vinegar in it to help set the whites. Crack the eggs into individual cups and place them in the water one at a time. The deeper the pan, the better, as the egg tumbles down through the water, it turns the white around the yolk, and if the water is deep enough, it will return to the surface without ever having touched the bottom. Once cooked (the yolks should still be very soft just as the whites are set), then plunge the eggs into iced water. They can be kept for a couple of days like this; to use, just pop them back into hot water for a minute to reheat.
4 portions
Peel the potatoes and parboil them in salted water; they want to have started cooking but still be very firm. Drain and allow to cool. Once cool enough to handle, grate them into a bowl. Heat 15 g/½ oz of the butter in a pan and gently fry the chopped onion without colouring until soft. Mix the onions, the melted butter and seasoning into the potatoes and form them into four small cakes about 7.5cm/3in across and 2cm/¾in deep.
Plunge the langoustines into boiling salted water for 45 seconds, then allow to cool. Once cooled, shell the tails, reserving the shells for another recipe.
Blanch the skin and deseed the tomatoes; cut the flesh into a 1 cm/½ in dice.
Heat the oil in a frying pan and add the potato cakes; gently fry these until golden brown on both sides. Meanwhile, heat 15 g/½ oz of the butter in another frying pan and quickly cook the langoustines in this for 1½ minutes; remove and keep warm.
Bring the tomato stock to the boil, and just before boiling, gradually whisk in 55 g/2 oz of the butter, whisking continuously until all the butter has melted. Hold the sauce hot but do not allow it to boil. Reheat the poached eggs for one minute in hot water. Place a potato cake in the centre of each plate, drain the eggs and place an egg on each potato cake. Position three langoustines around each cake; add the tomato dice to the sauce, then spoon the sauce over the egg and the langoustines. Garnish with a few sprigs of fresh chervil.