DOUGH: MAKE + CHILL (see recipe below): Make the dough as per instructions below (or use your own dough) and chill for an hour or freeze for 15 minutes.
PREHEAT THE OVEN to 425 degrees F and place oven rack at center. Line a baking tray with parchment paper and set aside.
STIR TOGETHER PLUM MIXTURE: While the pastry chills, in a large bowl stir together the sliced plums, sugar, and lemon zest. Set aside.
MAKE THE WALNUT STREUSEL (see recipe below) and set aside.
ROLL OUT DOUGH: Sprinkle the counter or board lightly with flour. Roll out the chilled pastry dough into an 11-inch round, lifting/turning dough and dusting counter again once or twice to keep dough from sticking. Roll the dough up onto the rolling pin (or carefully fold it into fourths) and move the dough to the center of the parchment-lined rimmed baking sheet.
SPOON PLUMS INTO PASTRY: Spoon/pour the plum mixture into the center of the pastry, stopping about 1 ½ inches before the edges. Top evenly with the walnut streusel. Bring up the edges of the dough around the plums, folding or pleating gently as needed to go around in a circle. You’ll have about a 9 ½-inch crostata (9" after baking). Brush the dough carefully and lightly with the milk and sprinkle only the dough with reserved teaspoon of sugar. (This will help brown the crust.)
BAKE/COOL/SERVE: Bake for 25-30 minutes or until crust is crispy golden brown and the filling is bubbly. Don’t worry if some of the juices have escaped onto the parchment paper. Remove pan to cooling rack. When it has sat for 30-40 minutes cooling, move the crostata off the paper so that it sits directly on the rack to cool completely as follows: Move the pan off the rack. Using both hands, lift the parchment paper off the pan (keeping the crostata level) and place it back onto the rack with the pie still on it. Carefully slide the crostata off the paper and onto the rack itself while pulling the paper from underneath it. Cool completely or until just warm. Slice and serve warm or at room temperature.
Store well-wrapped on the counter for 2-3 days or in the refrigerator for a week. If it’s very warm and humid when you are baking, you might want to wrap and leave the pie in the fridge immediately after serving. We never have much crostata leftover, but I would probably share it with neighbors or eat it for breakfast rather than freeze it. The dough, however, freezes very well if you’d like to make a double batch and freeze one. Let it sit overnight in the fridge, wrapped, to thaw before using.