Step 1

Roll out a disk of dough on a lightly floured surface to a 13" round (about ¼" thick). Transfer round to a parchment-lined baking sheet and chill while you roll out remaining piece of dough. Decoratively cut out shapes in the center of the second round, staying several inches in from the edges (long triangles, circles, diamonds—whatever tickles your fancy). Stack on top of the first round of dough. Chill while you prepare filling.

Step 2

Preheat oven to 350°. Combine granulated sugar, cornstarch, and salt in a large bowl. Add plums, vinegar, and thyme and toss to coat fruit evenly in sugar mixture.

Step 3

Beat egg with 1 tsp. water in a small bowl just to blend. Lift parchment with cut-out round of dough onto work surface. Using your hands, or wrapping dough around a floured rolling pin if you're nervous, carefully transfer uncut round of dough to a 9" pie dish. Lift up edges and allow dough to slump down into dish. Trim edges of dough with kitchen shears to even out, leaving at least a 1 ½" overhang; brush edge with half of egg wash. Scrape in plum filling along with any accumulated juices in bowl, then, using parchment, carefully transfer cut-out round of dough to pie, placing over filling.

Step 4

Working your way around the circumference, press edges of dough together to seal. Crimp as desired. Brush dough with remaining egg wash; sprinkle with demerara sugar.

Step 5

Place pie pan on a foil-lined rimmed baking sheet (juices may bubble over—this is what the foil is for). Bake until crust is deep golden brown on top and bottom and juices are bubbling, about 1½ hours. Transfer pie to a wire rack and let sit at least 4 hours before slicing.

Step 6

Do Ahead: Pie can be made 2 days ahead. Cover and chill.