For crust: I recommend using the Three-Grain Butter Pastry, which I find highly leak-resistant. Prepare the dough as directed. Lightly butter a large baking sheet. Set aside. Roll the dough into a 12½ inch circle and place it on the sheet. Put the sheet and pastry into the refrigerator for a few minutes while you make the filling.

For filling: Preheat the oven to 400°F. Put the plums into a mixing bowl. Mix the sugar and cornstarch in a separate bowl and stir into the fruit. Stir in the lemon juice. Set aside. Remove the pastry from the fridge. Imagine an 8-inch circle in the center of the rolled out pastry. Sprinkle the cornmeal evenly in that circle. Lift the plums out of the juice (do not discard) with a slotted spoon and spread them evenly over the cornmeal. Lift the pastry by sliding a spatula or dough scraper under the edge, then fold it up over the fruit; you want about 2 inches of pastry overlapping the fruit. (If the pastry starts to break, it is still too cold. Wait a minute or two, then continue.) Work your way around the crostata; the pastry will form pleats. Pour the juice that is in the bowl over the exposed fruit. Dust the top of the fruit with a few pinches of nutmeg. Lightly brush the pastry with milk and sprinkle with sugar. Bake on the center oven rack for 20 minutes. Reduce the heat to 375°F and bake for 30 to 35 minutes more, until the fruit is bubbly and thickened. Check the crostata as it bakes. If juice breaks through the crust, sprinkle 2 or 3 additional teaspoons of cornmeal on the sheet at the breach to contain the juice. Thoroughly cool the crostata on the sheet on a rack. Serve warm or at room temperate.