Roll out the pie pastry to a rectangle that is slightly larger than a 10-inch by 15-inch jelly roll pan. Greasing the pan is unnecessary. Carefully lift the pie pastry onto the pan and press the crust down gently so that it fits nicely into the pan. Trim any overhang with scissors or a sharp knife. Position the jelly roll pan so a long side is running parallel to the edge of the countertop. Scoop the first type of pie filling so that the left third of the crust is filled. Repeat with the second pie filling in the center third, then the third pie filling in the far right third. Scatter the crisp topping evenly over the entire slab pie. Place the pan on a sheet of aluminum foil that is slightly larger than the pan (to contain any potential bubble-overs) and bake for 40-50 minutes, or until the filling is bubbly, the crisp topping has begun to brown, and the visible pie crust is golden brown. Let rest until room temperature before cutting.