Purée all ingredients with 1 1/4 cups water in a blender until smooth. Set a fine-mesh strainer over a 9x9x2" metal baking pan; strain, discarding solids. Freeze mixture for 1 hour. Stir, mashing any frozen parts with the back of a fork. Cover and freeze mixture until firm, about 2 hours. Using a fork, scrape granita vigorously to form icy flakes. DO AHEAD: Can be made 3 days ahead. Cover tightly with foil and keep frozen. Give it a quick scrape before serving.