– Puree watermelon with lemon juice in a food processor. Force the puree through a fine sieve into a bowl to remove seeds and fiber.
– Whisk together sugar and gelatin in a medium bowl. Pour boiling water over the sugar mixture, stirring to dissolve. Add to the watermelon juice and mix well. Chill until cold, about 30 minutes. Pour into the canister of an ice cream maker and freeze according to the manufacturer’s directions. (Alternatively, freeze the mixture in a shallow metal cake pan or ice cube trays until solid, about 6 hours. Break into chunks and process in a food processor until smooth.)