Place the watermelon chunks in a single layer on a baking sheet and freeze for at least 4 hours or until completely firm.
Add the frozen watermelon chunks, lemon juice, and sugar to a blender or food processor.
Blend on high speed until the mixture is smooth and has a sorbet-like texture, stopping to scrape down the sides as needed. The mixture should be thick and creamy, with no large chunks remaining.
If using an ice cream maker, pour the mixture into the machine and churn according to manufacturer instructions until it reaches a soft-serve consistency.
Transfer the churned sorbet into a container and freeze for an additional 2 hours for firmness, or serve immediately for a softer texture.
Scoop the sorbet into bowls or cones, garnish if desired, and enjoy immediately for a refreshing finish.
