Chop the watermelon peel in to medium sized pieces. Heat a cooking pan with oil and cumin seeds. Include chopped watermelon peels too and saute for 3 to 5 minutes.
In a mixer jar include roasted watermelon pieces, grated coconut, green chillies and salt. Adding little water grind to make fine puree. Transfer it to a bowl and mix with buttermilk.
Heat a seasoning pan adding oil and mustard seeds, as the seeds crackle transfer the seasoning to prepared tambli.
Watermelon peel tambli is ready to relish with steamed rice.
Notes
It's not necessary to roast the watermelon peel for long. You can roast it for just 3 to 5 minutes.
If you don't have buttermilk, you can whisk yoghurt with some water and use.
Fresh buttermilk or whisked yoghurt gives best results than sour one.