1. In a large non-reactive pot, add the grapes along with the water. Cover, stirring from time to time, and cook until the grapes are soft.
2. Remove from heat and pass the grapes through a food mill with the attachment with the smallest holes. If you don’t have a food mill, press them through a fine-mesh strainer, which will take a bit of force. Different grapes will yield differing amounts of juice.
(I got 3 cups, 750ml, of juice from mine, which is good to know if you want to make this with high-quality bottled grape juice.)
3. Add the sugar, corn syrup and rosé, if using, and stir until the sugar is dissolved. Chill thoroughly, then freeze in your ice cream maker according to the manufacturer’s directions.