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.
Preheat oven to 400°F. Put plums into a bowl.
In a separate bowl, mix sugar and cornstarch and stir into plums. Stir in lemon juice. Set aside.
Remove pastry from the refrigerator. Imagine an 8-inch circle in the center of the rolled-out pastry. Sprinkle cornmeal evenly in that circle. Using a slotted spoon, lift plums out of juice (do not discard juice) and spread evenly over cornmeal. Lift pastry by sliding a spatula or dough scraper under the edge, then fold it up over the plums; you want about 2 inches of pastry overlapping the plums all around. (If 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 juice from the bowl over the exposed plums. Dust the top of the plums with a few pinches of nutmeg. Lightly brush pastry with milk and sprinkle with sugar. Bake on the center oven rack for 20 minutes. Reduce heat to 375°F and bake for 30 to 35 minutes more, until plums are bubbly and thickened. Check 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 juice. Thoroughly cool crostata on the baking sheet on a cooling rack. Serve warm or at room temperature.