Preheat to the oven to 375°F and line a rimmed sheet pan with parchment paper.
Roll the crust out on a lightly floured surface until it is a rough 12-inch circle. In a large bowl, toss together the cherries, cornstarch, sugar, lemon zest and lemon juice. Dump into the center of the circle, leaving a few inches around the edges.
Fold the edges up over the cherry mixture, pleating the dough as you go, until you've created a rim around the whole galette. Brush with egg wash and sprinkle with coarse sugar.
Bake until crust is golden and filling is bubbling, about 35-45 minutes. Let cool completely. Serve at room temperature or reheat to serve warm.