Sift the flour onto a pastry board or in the bowl of the mixer fitted with the paddle attachment;
Add salt, sugar, the diced cold butter, and mix until a crumbly texture;
With the mixer running, slowly add the slightly beaten whole egg and the yolk;
Add the lemon zest (grate at this time),
Transfer the dough on the pastry board and knead it quickly until it starts to come together. Watch out: it must not become too sticky, elastic or compact. If it is sticky, add some more flour. Knead until a smooth ball forms;
Wrap it into cling film and refrigerate for two hours or overnight in the lowest compartment of the fridge;
Butter and flour a round baking pan;
Roll 2/3 of the dough; a cold marble board would fit this operation the best, but if you don’t have one, roll the dough on a sheet of parchment paper;
Sprinkle the surface and roll to 1/8’’ thickness,
Gently roll the dough onto the rolling pin and transfer it to the baking pan;
Press the dough to the sides of the pan;
Trim the dish of extra dough;
Prick the dough with the tongs of a fork;
Evenly spread the jam in the shell; level it with a spoon. Watch out: do not fill it until the top of the shell, otherwise the jam will unpleasantly overflow;
Let the shell sit in the fridge;
Make a lattice pattern to decorate: knead the remaining dough and roll it out. Make sure the shortbread is cold and compact, then cut strips with a pastry wheel. If the dough is too soft or warm, refrigerate it for 20 minutes otherwise the strips will fall apart;
Cut from 6 (3+3) to 9 (4+5) 1(1,5) cm thick stripes;
Place the strips onto the shell: form a bottom layers of evenly spaced strips and then criss-cross with more strips. Gently press together and trim away the extra strips;
Pre-heat the oven at 180°C (356°F); in the meanwhile let the cake sit in the fridge again; it will keepsits shape more easily;
Bake for about 30 minutes until golden: it mustn’t get too dark,
Let it cool down (no less than 20 minutes) and remove the crostata from the baking pan;
Serve it completely cool when all the flavors have combined evenly.