First mix flour, butter, salt and an egg and work into a firm dough. Wrap the dough in cling film and put it in the refrigerator for half an hour. Take it out of the fridge, roll it out into a flat disc cca 1 cm thickness, which we spread in a cake tin. We press the dough into the edges of the form with our fingers and cut off the protruding parts with a rolling pin (if using a higher cake tin, we make the edges about 3 cm height). Prick the bottom with a fork and put it in the fridge for a while. In the meantime, set up the convection oven and pre-bake the dough in the first step. Then remove from the convection oven and mix the ingredients for the filling. Pour onto the body and bake on the second step of the program until the edges of the dough turn pink and the filling hardens. Leave the cake to cool in the tin and only then cut it.