Stir granulated sugar and lemon juice into the watermelon juice until the sugar is fully dissolved. Stir in Moscato.
Place the bowl or baking dish in the freezer and freeze the mixture for 2 to 3 hours, stirring with a fork every 45 minutes or so to break up ice crystals.
When ready to serve, spoon watermelon slush into a wine (or other) glass, filling the glass about 3/4 full. Pour in additional wine and stir. Garnish as desired.
* Six cups watermelon cubes, pureed and strained, yields about 4 cups watermelon juice. You can use watermelon juice released and collected when cutting up a whole watermelon instead, for all or part of the pureed juice.
