For the sour cherry jam, combine the sour cherries, sugar, and cinnamon stick in a pan and bring to a boil. Remove the cherries with a slotted spoon or skimmer to a bowl and slowly, over low heat, reduce the syrup until it is thick, about 10 to 15 minutes. Remove the cinnamon stick, return the cherries to the syrup, and cook until virtually no liquid remains. Pour the jam into a bowl and cool it.
For the pasta frolla, combine the dry ingredients and mix well. Rub in the butter until it is absorbed, making sure the mixture remains cool and powdery and does not become pasty. Beat the eggs and stir in with a fork. Continue stirring until the dough holds together, then knead it briefly, just until smooth. Shape into a disk, wrap in plastic, and refrigerate at least 1 hour, or until firm.
Preheat the oven to 350 degrees and set a rack in the middle level. Line with parchment a cookie sheet that has no rim on at least one side. To form the crostata, divide the pasta frolla in half and roll out one half into an 11-inch disk on a floured surface. Transfer the disk of dough to the lined cookie sheet and place a pattern on the dough, trimming it to a perfect 11-inch disk. Spread the jam on the dough to within 1 inch of the edge. Roll out half the remaining dough into a rectangle about 8 × 12 inches and cut it into ½-inch-wide strips. For the egg wash, whisk the egg with the salt; paint the strips with it and place them over the filling, 6 or 7 in each direction, in a diagonal lattice. Roll the remaining dough into a thin cylinder about 30 inches long. Egg-wash the rim of the pastry, then press the cylinder of dough onto it. Flatten the cylinder and score it at 1½-inch intervals with the bowl of a teaspoon and the back of a knife. Egg-wash the border. Chill the crostata at least 1 hour before baking it.
Bake for about 30 minutes, until deep gold. Be careful that the filling does not come to a boil during the baking. Slide the crostata off the cookie sheet by pulling on the parchment, and place it on a rack to cool.