Cook the split peas in the water in a covered pan for 30 minutes, or until nearly done.
Heat the oil in a large saucepan and fry the onion with a bit of salt until nicely brown. Remove the onion, then add the cumin seeds and fry for a few seconds. Add the split peas and their cooking liquid. Add the chillies, pumpkin, a bit more salt and cover. Simmer, stirring often, until the pumpkin is soft. Add the curry leaves.
Heat the mustard seeds in a dry frypan until they crackle and add to the pumpkin. Serve with the fried onion on top.