Prepare the pastry:
1. In a food processor, combine the flour, granulated sugar, and salt and pulse to mix.
2. Add the butter and pulse until the mixture is the texture of coarse meal with some pea-size pieces.
3. Add 6 tablespoons of ice water and pulse until the dough is evenly moistened. The dough should hold together when squeezed, but not be too wet.
4. Add up to 3 more tablespoons of water, if necessary.
5. Divide the dough in half, wrap each portion in plastic wrap, and form 2 rectangles.
6. Chill for at least 1 hour or up to 2 days. (alternatively, you can freeze the dough for up to 1 week.)
Prepare the filling:
1. In a medium saucepan, combine the golden raisins, raisins, rum, and 1 cup of water and bring to a boil over medium-high heat.
2. Cook, stirring occasionally, until the raisins are plump and the liquid has reduced by about half, about 6 minutes.
3. Add the brown sugar and continue to cook until the liquid is syrupy, about 4 minutes. Stir in the cornstarch and cook for 1 minute longer.
4. Remove from the heat and stir in the lemon zest and salt.
5. Transfer the raisin mixture to a plate, cover with plastic wrap, and transfer to the fridge to cool.
6. Preheat the oven to 400°f.
7. On a lightly floured surface, roll out 1 portion of dough to a 12 x 10-inch rectangle.
8. Transfer to a piece of parchment paper on a work surface.
9. Repeat with the remaining portion of dough.
10. Top 1 dough rectangle with the raisin filling, spreading it out to an even layer and leaving a 1-inch bare border all the way around.
11. Sprinkle the butter evenly over the top.
12. Brush the beaten egg on all 4 edges.
13. Top with the remaining rectangle of dough.
14. Use a fork to crimp the 4 edges together.
15. Use a sharp knife to trim about 1/8 to 1/4 inch off each edge.
16. Use the parchment paper to lift the pie onto a rimmed baking sheet.
17. Cut 4 small slits in the top sheet of dough.
18. Cover and freeze the pie for 15 minutes.
19. Brush the top of the pie with beaten egg.
20. Bake until deep golden brown and crisp, about 40 minutes.
21. Transfer the pie, on its baking sheet, to a wire rack to cool.
22. Serve warm or at room temperature with ice cream.
23. This pie is best the day it’s made but leftovers can be stored at room temperature for up to 3 days.