Put in fresh mint leaves, fennel seeds, whole pepper, chaat masala and salt to a small mixer jar and grind to a fine powder. Set aside. Make sure your mint leaves are not wet.
Into a juice jar / blender put in the chopped watermelon cubes, jaggery and 1/4 cup water. Blend well to get the juice.
Add the prepared spice powder and lemon juice to the watermelon juice and mix well. Pour the chatpata watermelon juice to glasses and serve chilled by adding ice cubes. (if using)
You can squeeze in more lemon juice if required.
You can increase the quantity of jaggery powder as per your liking or skip completely if your watermelon is sweet enough. You can use sugar instead of jaggery, if you wish to.
If you do not have fennel seeds (saunf) you can add 1/4 tsp of cumin seeds. The taste of the juice might vary slightly.
You can strain the juice to remove the pulp and then serve, if you wish to. However, I do not prefer to strain and hence, I haven't mentioned this step in the recipe.