Preheat oven to 400°F (200°C). Roll puff pastry until it's 1/8 inch thick. Using a bowl as a template, use the tip of a sharp knife cut out a 10 1/2 inch circle of puff pastry.
Over medium-high heat, melt butter in a 10-inch ovenproof skillet, preferably non-stick or cast-iron. Stirring frequently, add grapes and cook until many 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 grapes into a single layer and cover with puff pastry. Bake until puffed and brown, about 15 minutes. Let cool 5 minutes, then carefully turn out onto a round plate. Serve warm and cut into wedges. Top with whipped cream if desired.