Begin by making the jam. Place the blackberries, sugar, ground rosemary and lemon juice plus zest in a large pot over medium low heat. Then, crush them with a potato masher.
This step is optional. If you prefer to remove some of the pulp, remove the fruit by the spoonful and pass it through a fine mesh strainer, pressing down on the fruit, allowing the juice to return to the pot, while the solids get left behind.
Add the pectin, a little at a time, stirring until it has completely absorbed into the mixture.
Bring the mixture to a boil and then reduce heat and allow the jam to simmer, uncovered, for about 25-30 minutes, stirring occasionally. Then, remove it from the heat and allow it to cool for an hour. It will continue to thicken as it cools.
To make the tart shell, place the flour, salt and sugar in the bowl of a food processor fitted with a steel blade and pulse a few times to mix. Add the butter and shortening. Pulse until well combined, but do not over mix. With the machine running, pour ice water down the feed tube and pulse the machine until the dough comes together.
Remove the dough from the food processor and place it on a flat surface. Using a pastry cutter or a knife, cut the dough in half and form it into two disks. Wrap the dough in plastic wrap and refrigerate for 30 minutes.
After the dough has rested, roll out one disk between two sheets of parchment paper until it measures 12" in diameter, about 1/8" thick.
Press the crust into the tart pan and trim excess dough. Then, place a sheet of tin foil over the dough and weigh the top down with pie weights or beans. Bake for 20 minutes, remove from the oven and allow to cool slightly.
Then, spread the jam in an even layer over the tart shell, about 1/2" in thickness.
Roll out the rest of the dough and cut into strips about 1" wide. After cutting the dough into strips, I refrigerate them for about 20 minutes. It makes the dough much easier to work with if you plan to basketweave or braid the strips. Make a lattice and decorate the top of the tart.
Brush with an egg wash, dust with sugar and bake at 350°F for 25-30 minutes, until the pastry is golden and the filling is heated through. Allow to cool on a wire rack before serving.
