Heat oil in a pan. Add the urad dal and green chilies and saute.
Next, add the onions, green chilies, and coriander leaves. Saute. Let it cook for 3-4 minutes.
Add the chopped watermelon rinds next.
Add the salt next and mix.
Keep aside and let it cool down to room temperature.
Transfer the mixture to a blender along with the tamarind paste and blend into a smooth paste. You should be able to make the paste without adding water. If it turns too thick then add 1-2 tablespoon of water.
Add the tamarind paste and blend it once more.
Transfer the chutney to a bowl and keep aside.
Prepare the tempering by heating oil in a small pan and adding the mustard seeds and curry leaves. Once they start to crackle, immediately transfer and pour on top of the chutney. Mix.
Serve with dosas of your choice.