Instructions: Make shell: In a bowl with a pastry blender or in a food processor blend or pulse together flour, butter, shortening, and salt until mixture resembles coarse meal. Add 2 tablespoons ice water and toss or pulse until incorporated. Add enough remaining ice water, 1 tablespoon at a time, tossing with a fork or pulsing to incorporate, to form a dough. On a work surface with heel of hand smear dough in 3 or 4 forward motions to make dough easier to work with. Form dough into a ball and flatten to form a disk. Chill dough, wrapped in plastic wrap, at least 1 hour and up to 24. On a lightly floured surface with a floured rolling pin roll out dough into a 16- by 7-inch rectangle (about 1/8-inch thick). Fit dough into a 13- by 4-inch rectangular tart pan with a removable fluted rim. Chill shell, covered, 1 hour, or until firm. Preheat oven to 400 degrees. Line shell with foil and fill with pie weights or raw rice. Bake shell in middle of oven 10 minutes. Remove weight or rice and foil and bake shell until pale golden, about 5 minutes more. Cool shell in pan on a rack. Shell may be made 2 days ahead and kept in pan, covered, at room temperature. Reduce temperature to 250 degrees and transfer pan to a baking sheet. Make custard: Whisk together custard ingredients. Pour custard into tart shell.

Bake tart on baking sheet in middle of oven 30 minutes and transfer to rack to cool. Custard will continue to set as tart cools. Tart may be prepared up to this point 1 day ahead and chilled, covered. In a small saucepan cook 1 cup Boysenberries and sugar over moderately-low heat, mashing with a fork and stirring occasionally, 5 minutes and pour through a fine sieve into a small bowl, pressing hard on solids. Discard solids and cool Boysenberry mixture. Remove rim of tart pan and transfer tart to a platter. Pour cooked Boysenberry mixture over custard and arrange remaining 2 cups Boysenberries on top.

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