Put the flour in a bowl. Add cubes of fridge-cold butter.
Quickly work the flour and butter together until the mixture resembles fine breadcrumbs. Add the water and combine to form a firm dough. Press the dough out in a flan case or spring-form pan. Prick all over with a fork and refrigerate for 10–15 min. Fill the pastry-covered flan case with the scampi, cheese and leeks. Beat together the eggs and milk/cream. Season with salt and a little pepper. Pour the liquid carefully over the filling. Bake at the bottom of an oven preheated to 200 °C for 45 min. Leave to cool. Top with a whole langoustine and serve while still slightly warm, accompanied by a mixed salad.