In a large mixing bowl (or the bowl of a standing mixer) add the flour, sugar and salt. Mix well.
Add the butter and mix again, incorporating until you have a sand-like mixture. Then add the eggs and mix to form a proper dough, adding, if need be, add a spoonful or two of milk or water.
Wrap the dough and plastic wrap and let it sit in the refrigerator for at least 30 minutes.
Preparing the ricotta cream filling
In another mixing bowl, whisk the ricotta and sugar (and the egg, if using) vigorously until you have smooth cream. Set aside until needed.
Forming the bottom crust
When the dough has rested, take it out of the fridge. Divide it into two balls, with one ball slightly larger than the other.
Roll out the larger bowl into a round about 30cm (12 inches) in diameter.
Lay this round onto your baking vessel a 20cm (9 inch) tart dish or pie plate, making sure to cover the bottom and sides. If using a springform pan, this bottom round should come at least halfway up the sides. Trim off any excess. Puncture the bottom here and there with a prongs of a fork.
Filling the crostata
Slather the sour cherry jam to cover the bottom of the crust. Then pour over the ricotta cream and spread it out over the bottom with a spatula.
Forming the top crust
Now roll out the other ball of dough into a second round just wide enough to cover the tart dish or pie plate.
Lay this second it over the filling, Trim off any excess and then pinch it around the edges to seal the top and bottom crust together.
Baking and serving
Bake your crostata in a moderate (180C/350F) oven for about 45 minutes, until cooked through and nicely brown on top.
Let your crostata cool completely and rest for at least two hours.
Unmold your crostata on to a serving platter and serve, dusted with confectioner's sugar if you like.