Preheat the oven to 400 F. Roll the puff pastry into a 1/8-inch thickness. Using a bowl as a template, with the tip of a sharp knife cut out a 10 1/2-inch circle of puff pastry.

In a 10-inch ovenproof non-stick or cast-iron skillet, melt the butter over medium-high heat. Add the grapes and cook, stirring frequently, until many of the grapes have burst and are golden in places – about 15 minutes. Stir in the sugar, lemon and salt, and simmer 1 minute longer or until liquid has thickened slightly. Push the grapes into a single layer and cover with the puff pastry. Transfer to oven and bake until puffed and brown, about 15 minutes. Let cool 5 minutes, then carefully turn out onto a round plate. Serve warm, cut into wedges. Top with whipped cream, if desired.