Peel and slice the apples. Bring 1 cup water, sugar, and the lemon zest to a boil. Simmer the apples, currants and raisins in the syrup until the apples become transparent and the currants and raisins are swollen, about 10 minutes. Drain fruit, reserving fruits and liquid, and then spread the fruit out on a metal pan so it will cool quicker.
Place flour, sugar, and salt in a mixing bowl and refrigerate to chill. Cut the butter into small pieces and freeze until firm (or freeze the butter and then grate). Using the flat paddle of a mixer, or by hand, cut the butter into the flour, mixing until the bits resembles oatmeal. Add the egg and mix just long enough for the dough to form a rough ball, adding a few teaspoons of ice water only if necessary. Pack the dough into a rectangular shape in a plastic bag and chill at least 1 hour or up to 2 days. Roll about 2/3 of the dough on a floured surface to a sheet a bit more than 1/4" thick and chill. Roll the remaining half of the dough the same way and chill. From the first sheet of chilled dough, cut out a round piece large enough to cover the bottom and 1 1/2" up the sides of a 9" x 2" springform pan. Roll out the remaining chilled dough and cut into strips for the lattice top (using a pinked cutter if possible). Chill the strips while assembling the cake.
Fill the pastry-lined pan with the cooled apple/currant mixture. Arrange the top layer of apples in a nice pattern. Make a lattice top to cover the top. With the remaining dough strips, form a border and pat onto the edge of the tart. Brush the lattice and the rim with the egg wash. Place the cake on a baking sheet and bake at 325°F until lightly browned with bubbling filling, about 1 hour. Remove the cake from the oven and allow to cool. Meanwhile, boil the reserved apple syrup until thick and syrupy and just beginning to darken in color, then pour over the cake while warm, but not hot. Cool before cutting into 8 to 12 wedges.