Mix the butter, flour, salt and sugar in a bowl. Work quickly. Add the egg yolk and a little water. Work lightly. Add a little water if the dough, which must remain firm, crumbles.
Flour and let stand for 1/2 hour.
Line a pie tin, prick the dough over its entire surface and blind bake for 20 minutes (th. 7).
Meanwhile, separate the whites from the yolks. Work the yolks with half the sugar to obtain a foamy cream. Add the grated lemon zest, the juice of 2 lemons, the flour, working with a wooden spoon. Cook this cream in a bain-marie, and stir it until it thickens (about 15 minutes).
Remove from heat.
Beat the egg whites very stiff. Add the rest of the sugar to them. Pour everything into the cream in small quantities, incorporating gently.
Fill the baked dough. Return to hot oven for 10 to 15 minutes (th. 7). The cream rises slightly and falls a little as it cools.
Source:Les Bons Desserts. Claire Mignieres. Baudouin.
