For the dough: In a food processor, combine the flour, cornmeal, and salt. Pulse until combined. Add the butter and pulse until the mixture is crumbly and the butter is the size of small peas. Add the sour cream and pulse to combine. Add the water, 1 Tbsp. at a time, and pulse just until the dough begins to form a ball. On a lightly floured board, form the dough into a ball, divide into 6 portions, and shape each into a disk. Wrap in plastic wrap and refrigerate for 30 minutes.

For the syrup: In a small saucepan, combine the sugar, wine, water, vinegar, rosemary, and lemon zest. Bring to a boil over medium heat, reduce the heat to a simmer, and cook until reduced and thickened to a syrup, 25-30 minutes. Remove from the heat and let cool. Strain through a fine-meshed sieve into a sealable container.

Preheat the oven to 375°F. Line a baking sheet with parchment paper.

On a floured board, roll out each pastry disk into a 6” round, about 1⁄4” thick, and place on the prepared pan. Arrange an equal amount of plum slices in concentric circles in the center of each disk, leaving a 1” border. Fold the border over the plums, pleating as you go. Brush each pastry with the egg. Sprinkle each with about 1 tsp. of the raw sugar and bake until the pastry is golden brown, about 30 minutes. Remove from the oven and transfer to wire racks. Let cool completely.

Arrange the crostadas on a platter or divide among 6 small plates and drizzle with the rosemary syrup. Serve.