Cut pumpkin in small long pieces like you would cut raddish for salad.
Soak mustard seeds for 1/2 hour (this makes grinding easy and does not make the paste bitter). Gring mustard seeds to a smooth paste with water.
Heat Oil in kadai. When oil is hot, add onion seeds. You will get a fragnance at that time add the pumpkin. Stir in oil for 5 mins on slow flame.
Add salt, turmeric and chilli powder. Stir for another 5 minutes.
Add a little water to the mustard seeds and add it to the vegetable through a sieve (this will avoid the unground pieces of mustard like the skin of the seed from going into the vegetable).
Then add the grated coconut. Mix thoroughly.
Cover and cook on slow flame till pumpkin is soft. Do not add water.