Combine lemon juice and simple syrup in an 8-by-8-inch glass baking dish. Transfer to freezer and freeze, raking with a fork every 30 minutes, until flaky and frozen, about 3 hours.
Transfer to a blender and puree with 1 cup ice-cold water. Divide among five 5-ounce ramekins or disposable paper cups and freeze until firm, at least 4 hours.
Remove from freezer 15 to 20 minutes before serving.
