Cut 500 grams of watermelon into small cubes, removing the seeds. Keep the watermelon cubes in the refrigerator for 30 minutes to chill.
In a blender, add the chilled watermelon cubes, 250 grams of chilled plain yogurt (dahi), 2 tablespoons of sugar, and a pinch of salt. You can adjust the sugar according to the sweetness of the watermelon.
Blend the mixture until smooth and frothy.
Pour the watermelon lassi into glasses and garnish with a few mint leaves or a sprinkle of chaat masala for an extra zing.