Start by preparing your watermelon. Cut the watermelon into cubes, ensuring to remove any seeds. Spread the cubes in a single layer on a parchment-lined baking sheet or shallow dish and freeze for at least 3-4 hours, or until completely solid. This step is crucial for the sorbet's texture.

Once the watermelon is frozen solid, transfer the frozen cubes to a powerful blender or food processor. Add the freshly squeezed lime juice and, if desired, 2 tablespoons of agave nectar or honey. Start blending on low, then gradually increase to high speed, using a tamper if your blender has one, or stopping to scrape down the sides as needed.

Continue blending until the mixture is smooth and creamy, with no ice chunks remaining. Taste and add more agave nectar or honey if you prefer it sweeter. The mixture should be thick, similar to a soft-serve consistency.

Pour the blended watermelon mixture into a shallow freezer-safe dish (a glass baking dish or metal pan works well). Cover the dish tightly with plastic wrap or a lid.

Place the dish in the freezer. To achieve the best sorbet consistency, stir and scrape the sorbet with a fork every 30-45 minutes for the first 2-3 hours. This helps break up ice crystals and keeps it smooth. After this, let it freeze for another 1-2 hours or until firm enough to scoop.

Before serving, if the sorbet is too hard, let it sit at room temperature for 5-10 minutes to soften slightly. Scoop into bowls or glasses and serve immediately. Garnish with a fresh mint sprig or a small lime wedge, if desired.