for the pastry
2 1/2 cups all purpose flour
1 tablespoon sugar
3/4 teaspoon salt
10 tablespoons (1 1/4 sticks) chilled unsalted butter, cut into 1/2-inch pieces
1/3 cup chilled solid vegetable shortening, diced
6 tablespoons (or more) ice water
for the filling
10-12 plums, sliced
1/2 cup fresh blackberries
1 tablespoon corn starch
1/2 cup sugar
1 egg yolk mixed with 1/4 cup milk
1/3 cup sliced almonds
Blend flour, sugar and salt in processor. Add butter and shortening and cut in using on/off turns until mixture resembles coarse meal. Add 6 tablespoons ice water and process until moist clumps form, adding more water by teaspoonfuls if dough is dry. Gather into ball, divide, then flatten into two disks. Wrap in plastic; chill 2 hours.
Preheat oven to 375F. Spray a 9 inch pie plate with baking spray.
Slice plums and toss with blackberries in a medium bowl. Sprinkle in corn starch and sugar and stir to combine.
Roll out one of the dough disks to a 12 inch circle. Place in pie plate and crimp edges. Dock the pastry with a fork a few times, all the way to the edges. Spoon plum blackberry mixture onto the pie plate. Roll out the other dough disk to a long sheet and cut bands. Create a lattice top over the pie and anchor to the crust. Lightly brush pastry with some of beaten egg wash.
Bake until filling is bubbling and pastry is golden, 55 minutes to an hour. Cool slightly on baking sheet on a rack. Serve warm or at room temperature.