Crust: Combine the flour, cornmeal, sugar, lemon zest, salt and chopped rosemary in the bowl of a food processor fitted with a steel blade.

Pulse a few times to mix.

Add the cold, chopped butter and pulse about 10 -12 times until the butter is cut into the flour mixture and is pea size.

Pour in the ice water and pulse a few more times until a ball begins to form, being careful not to overmix. The dough should not be completely blended, it will come together on the counter.

Scrape dough onto counter and form into a flat disc. Wrap in plastic wrap and refrigerate for at least an hour or overnight. (Dough can also be frozen for several weeks. Defrost in refrigerator before proceeding.)

Preheat oven to 400 degrees and remove dough from the refrigerator.

Roll out dough between lightly floured pieces of wax paper, carefully turning and lightly flouring to prevent sticking. Roll into a 16 inch circle, don't worry if the edges are a little craggy.

Move the pastry round onto a parchment lined sheet tray.

Filling: Whisk together the mascarpone, lemon juice, vanilla, yolk and heavy cream in a medium bowl.

Crostata: Pour mascarpone filling into the center of the pastry round in roughly a 10 inch circle, leaving about 3 inches of dough exposed on the edge.

Sprinkle the grapes over the mascarpone mixture and scatter rosemary over the grapes.

Fold and pleat the border over the filling, leaving about 6 inches in the center exposed.

Place the unbaked crostata into the refrigerator for 20 minutes to chill.

Remove and brush the edges with cream and sprinkle the turbinado sugar over the edges.

Bake for about 50 mins or until golden brown and filling is set. If the edges are getting too dark before the filling is set, cover loosely with foil.

Remove from the oven and let cool on the sheet tray for 15 minutes before carefully removing to a wire rack to finish cooling.

Sprinkle with powdered sugar if desired and serve. Crostata can be served warm or room temperature.