Combine the flour, sugar, butter and egg yolks to make smooth dough. Roll out and place in a 35×25 cm baking tin. Bake on the middle rack, at 160 °C, about 20 minutes, until the rim starts “shrinking” but the top is half-done.
Take out of the oven and spread a thin layer of jam over the top.
Beat the egg whites until firm peaks form. Add the sugar and beat until firm. Gently stir in the walnuts.
Spread the mixture over the crust. Place on the bottom rack of the oven preheated to 100 °C (so that it can slowly dry). When half dried, take out of the oven and slice half way through (only the egg white part) to the serving size. Return to the oven (at 100 °C) for up to one more hour. Take out and cut as marked.