Put the flour on your work surface, pile it up, create a “well” in the centre and place in it the egg yolks, the softened butter (take it out of the fridge at least 20 minutes before) cut into pieces, the sugar, the grated lemon peel and a pinch of salt. Mix everything together rapidly, with cold hands; if the dough sticks to them too much, use a little flour. Form a smooth ball, wrap it in cling film and put it in the fridge to rest for at least 2 hours.
In the meanwhile prepare the filling for the tart. Wash and peel the apples, remove the core and slice them as thinly as you can. Put the slices in a bowl with the lemon juice and 2 tablespoons of brown sugar, mix well and leave to stand. In another large bowl, use an electric whisk to beat the egg yolks with 6 tablespoons of sugar, until the mixture becomes pale and fluffy. Add the mascarpone (previously kept at room temperature for about half an hour) and carefully mix it with the beaten yolks, using a wooden spoon, until you have a cream that’s smooth, even and fairly thick.
Take the pastry out of the fridge (set aside roughly a quarter, which you will use later in strips on top of the filling) and use your hands to spread it out in the baking tin, which you have previously greased and floured, making sure the sides are covered to a height of 3 cm. Prick the pastry bottom here and there with a fork, so that it doesn’t rise while cooking. Arrange the slices of apple in the bottom, one beside the other, overlapping slightly (like a fan), pour the mascarpone cream on top and level the surface. Sprinkle a handful of flour on the work surface and, with a rolling pin, roll out the pastry you set aside; use a pastry wheel to cut it into strips and lay them on top of the tart to create the decorative lattice. Bake in a preheated oven, at 180 °C, for 50 minutes until the surface is golden brown (If you see that it’s browning too much, cover it with a sheet of kitchen foil).Take it out of the oven and leave to cool down completely before serving.
It’s a simple, very tasty tart; if it doesn’t “disappear” immediately (because it’s delicious) it’s even better the next day. So if you have guests, you can prepare it in advance.