1. Prepare the Watermelon: If you haven’t already, cut your seedless watermelon into 1-inch cubes. Spread the cubes in a single layer on a baking sheet lined with parchment paper. Place the baking sheet in the freezer for at least 4 hours, or until the watermelon is solid. This step is crucial for the slushie texture.

2. Gather Your Ingredients: Once the watermelon is frozen, retrieve it from the freezer. Have your fresh mint leaves, lime juice, cold water, and optional sweetener ready.

3. Add to Blender: Carefully transfer the frozen watermelon cubes into a high-speed blender. Add the fresh mint leaves, fresh lime juice, and cold water.

4. Start Blending: Begin blending on a low setting, gradually increasing the speed. You may need to use the tamper attachment (if your blender has one) or stop and scrape down the sides of the blender with a spatula a few times to help incorporate all the ingredients.

5. Achieve Slushie Consistency: Continue blending until the mixture is smooth and has a thick, slushie-like consistency. There should be no large chunks of watermelon remaining.

6. Taste and Adjust Sweetness: Pour a small amount into a spoon and taste. If desired, add 1-2 tablespoons of granulated sugar or honey and blend again briefly until fully incorporated.

7. Serve Immediately: Pour the watermelon mint slushie into chilled glasses. Garnish with a fresh mint sprig or a small wedge of lime, if you like.