1. Line a baking sheet with parchment paper or a nonstick baking mat.

2. Sprinkle a lightly floured work surface with 1 tablespoon turbinado sugar. Roll out dough to a 12-inch round, about 1/4-inch thick. Transfer dough to prepared baking sheet and refrigerate until ready to use, up to 1 hour. Meanwhile, preheat oven to 375 degrees.

3. In a small bowl, mix together 1 tablespoon flour, granulated sugar, cinnamon, and nutmeg. In a large bowl gently toss together plums and zest. Sprinkle the flour mixture over plums; gently toss until evenly coated. Transfer plum mixture on top of dough, leaving a 2-inch border all the way around. Fold border over plum mixture, overlapping where necessary and gently pressing to adhere the folds.

4. In a small bowl, beat together egg with 1 teaspoon water. Brush edges of dough with egg mixture, and sprinkle with turbinado sugar. Bake until crust is golden brown and juices are bubbling, about 45 minutes. Transfer the baking sheet to a wire rack to cool. Serve warm or at room temperature.