Prep: 1. Roll out both balls of pie dough. In one, cut out a few decorative shapes with a knife or small cookie cutter (this will be the top). Place on parchment and refrigerate while preparing filling. 2. Pit and slice plums. Chop thyme. 3. Preheat oven to 375. Make: 1. Combine granulated sugar, cornstarch, and salt in a large bowl. Add plums, vinegar, and thyme and toss to coat fruit evenly in sugar mixture. 2. Beat egg with 1 teaspoon water in a small bowl just to blend. 3. Transfer the pie dough without the cutouts to a pie dish, leaving a 1 1/2 inch overhang on all sides (cut off any additional excess) and brush with the egg wash. 4. Pour in plum mixture and top with second piece of pie dough (with cutouts), again leaving a 1 1/2 inch overhang. Press together the edges and decoratively crimp if desired. Brush the top with remaining egg wash and sprinkle with raw sugar. 5. Place pie pan on a foil-lined rimmed baking sheet (juices may bubble over). Bake until crust is deep golden brown on top and bottom and juices are bubbling, about 1 1/2 hours. 6. Transfer pie to a wire rack and let sit at least 4 hours before slicing.
You can exclude the thyme if you prefer (but we recommend giving it a try!). If your plums are very sweet (or if you really like thyme) you can up to double the thyme.