Firstly, take a small blender and add chopped watermelon. Blend this to a fine puree. Keep this aside.
Heat a khadai or pan on a moderate flame and add ghee.
As the ghee starts to melt, add mustard, cumin, red chilli, hing and curry leaves.
Saute all the ingredients until it splutters.
Then add the blended watermelon puree. Mix it well.
Next add water as needed and give a quick mix.
Keep the flame low and add tamarind juice, salt, jaggery and rasam powder.
Mix very well and also add coconut milk.
Now bring this mixture to a boil for 4 to 5 mins, keeping the flame high.
Finally add chopped coriander leaves and turn off the heat.
Now the delicious watermelon rasam is ready to serve.
Serve this flavorful watermelon rasam with hot plain rice, with a dollop of ghee.