Preheat the oven to 400 degrees. Roll the dough (either store-bought or homemade) out into a 12-inch round (it can be ragged). Transfer to a rimmed baking sheet lined with parchment paper and chill while preparing the filling.
For the grape filling: Toss together the grapes, all but a tablespoon of sugar, the salt, the lemon juice and zest, honey, and the cornstarch. Pile fruit on the dough circle, leaving one 1/2-inch border. Gently fold the pastry over the fruit, pleating to hold it in (sloppy is fine).
In a small bowl, whisk together the egg and cream. Brush pastry generously with egg and cream mixture. Sprinkle remaining sugar on the crust. Bake for 35 to 45 minutes, until the filling, bubbles up vigorously, and the crust is golden. Cool for at least 20 minutes on wire rack. Serve warm or at room temperature.