Blackberry Slab Pie Bars are a little slice of heaven ~ the flakey pastry, the jewel toned filling, and the sweet glaze all work together in this super user friendly dessert. Use frozen fruit or preserves when fresh berries aren't available.
Start by making the crust. Pulse the flour and salt a few times in a food processor to combine. Add the cold chunks of butter and shortening to the bowl and pulse about 20 times until the mixture is grainy.
Add the ice water, a little at a time, while continuing to pulse, until the dough just comes together when you pinch it between your fingers. Stop the machine to check it. NOTE: you may not need all of the water, but I always do.
Turn the dough out onto a floured surface and bring it together into a ball. If there is still any remaining dry flour make sure to knead that into the dough to get it all incorporated. If your dough is too sticky, add a touch more flour.
Cut the ball of dough in half, form two flat rectangles, and wrap each one in plastic. Refrigerate the dough for 2 hours (or overnight) before rolling it out.
While the dough is chilling make the filling. This step can also be made the day before if you like. Put the berries into a saucepan. Mix the sugar and Clearjel thickener together and add to pan along with the lemon juice. Starting on low and then increasing the heat to medium high, cook the berries until they start to soften and the sugar has dissolved. Mash the berries with a fork as they soften. Stir almost constantly.
Cook for about 8 minutes, or until it's the consistency of a runny jam. I like to mash most of the berries. Chill the filling until ready to use.
Preheat the oven to 350F
Lay out a sheet of parchment large enough for a baking sheet and lightly flour it. Roll out one of the pieces of dough to (roughly) a 9x13 rectangle. Don't stress, if it's close, that's fine. It helps to let the dough rest for a few minutes to warm up slightly before trying to roll it out.
Spoon the jam over the top, leaving about an inch all around free.
Roll out the top crust, and carefully place over the jam filling, pressing down all around the edges to seal. if you've got excess dough you can roll it up a bit.
Brush with the egg wash and bake for about 40 minutes until golden. Let cool before glazing.
To make the glaze whisk or stir the sugar together with the water, a tablespoon at a time, until you get a glaze consistency. Brush it liberally over the top of the cooled pie. Let dry before slicing.