Step 1: In a large bowl, whisk together the flour, 1 tablespoon of sugar, and salt. Add the cold cubed butter and use a pastry cutter or your fingers to work the butter into the flour until it resembles coarse meal.
Step 2: Gradually add ice water, one tablespoon at a time, mixing until the dough just starts to clump together. Form into a disk, wrap in plastic, and refrigerate for at least 30 minutes.
Step 3: Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper.
Step 4: In a medium bowl, toss the sliced plums with 1/4 cup sugar and lemon juice.
Step 5: On a lightly floured surface, roll the chilled dough into a 12-inch circle. Transfer the dough to the prepared baking sheet.
Step 6: Sprinkle the almond flour or cornstarch over the center of the dough, leaving a 2-inch border. Arrange the plums over the almond flour.
Step 7: Fold the edges of the dough over the plums, overlapping where necessary. Brush the edges with the beaten egg and sprinkle with turbinado sugar.
Step 8: Bake for 30-35 minutes, or until the crust is golden brown and the plum juices are bubbling. Let cool slightly before serving.