STEP THREE: MAKE FILLING
In a large bowl, mix the apples, sugar, cornstarch, cinnamon and lemon juice together. Spoon the mixture into the crust-lined pan. Sprinkle a layer of raspberries on top. Then unroll and stack the remaining 2 pie crusts onto a lightly floured surface. Roll these into one 17x12-inch rectangle. Fit this rectangle on top of the apple mixture.
STEP FOUR: ADD TOP CRUST
Pinch the edges of the crusts together to seal, tucking any extra pastry crust under the crust, if necessary and crimp the edges. Cut several small steam holes on the top crust to allow steam to escape. Bake the slab pie for 45 to 50 minutes or until the crust is golden brown and the filling is bubbling. Cool it on a rack for 45 minutes, then cut it into 4 rows by 3 rows to serve.