Step 1Combine the sugar and water in a small saucepan and bring to a simmer. Simmer just until the sugar dissolves and turns clear, 1 to 2 minutes. Remove the pan from the heat and let the mixture cool to room temperature. Stir in the limoncello and lemon juice. Chill the mixture in the refrigerator until cold, 1 hour, then freeze in an ice cream maker according to the manufacturer's instructions.
Step 2If you're not using a machine, pour the chilled mixture into a 9-inch square metal baking pan and freeze. After half an hour, remove the pan from the freezer. Scrape down the sides and bottom of the pan with a metal spatula. Chop the frozen lumps into liquid, then return the pan to the freezer. Repeat the process every half hour until the ice becomes slushy. This should take about 1 1/2 hours.
Step 3Spoon the ice into glasses and serve. To serve the ice later, spoon it into a plastic container with a tight-fitting lid and return it to the freezer.