Preheat your oven to 425°F (220°C). Position the rack in the lower third of your oven for even baking.
Line a rimmed baking sheet with parchment paper to prevent sticking.
Make the almond-sumac mixture by pulsing the ground almonds and sugar in a food processor until it resembles coarse sand.
Transfer the mixture to a bowl and stir in the sumac.
Roll out the pie dough on a floured surface to a 12-inch round, about ⅛-inch thick, and transfer to the baking sheet.
Sprinkle the almond-sumac mixture in the center of the dough, leaving a 2-inch border.
Arrange sliced plums over the mixture, fanning them out.
Fold the edges of the dough over the plums, pleating gently.
Brush the folded dough edges with the beaten egg and sprinkle extra almond-sumac mix if desired.
Bake for 35 to 40 minutes until the crust is golden brown and the fruit is tender.
Allow to cool for 10 minutes before slicing. Serve warm or at room temperature.
