Place the ricotta to drain in a colander, as it must be perfectly dry and free of the whey (it is best to leave it overnight in the refrigerator). Remove the stones from the wild cherries and put them into a bowl.For the shortcrust pastry: Mix the sieved flour and the sugar and make a well in the mixture. Add the egg yolks, the cold butter cut into cubes, a pinch of salt, and the grated lemon rind. Work it all together quickly. To make the dough softer and to help it bind better, add an egg white. When the dough has come together, make it into a ball, wrap it in clingfilm and leave it to rest in the refrigerator for at least a couple of hours. Grease a baking tin well and sprinkle it with flour. Roll out the pastry to a thickness of 1cm and place it in the tin. Remove any pastry that hangs over the edges, cut it into strips and put it aside.
For the cream filling: Put the ricotta, sugar and eggs into a bowl. Add the lemon rind and stir well to obtain a creamy mixture.When the mixture is ready, spread it evenly over the shortcrust base in the baking tin, and then arrange the well-drained wild cherries, on top. (Alternatively, spread an even layer of wild cherry marmalade over the creamy mixture). Use the strips of remaining shortcrust pastry to make a crosshatch design on the top of the tart. Decorate the tart with whole cherries.Bake the tart in the middle of a pre-heated oven at 170 °C for about 40 minutes. While it is cooking, if the tart browns too quickly, cover it with aluminium foil. When it is ready the top of the tart should be lightly browned.Allow the tart to cool in its tin, then place it on a serving dish, dust with icing sugar and serve in thin slices.