Heat oil in a non stick pan. Peel 1 ridge gourd and do not discard the peels. Cut it into big cubes. Cut the other gourd into big cubes without peeling.
Add mustard seeds to the pan, and when they splutter, add curry leaves. Roughly chop onion and add along with green chillies and gourd peels and saute. Add salt and roasted chana dal and saute.
Add ridge gourd cubes and saute till they soften. Cool the mixture slightly and transfer into a mixer jar. Add tamarind pulp and grind till well blended. Transfer into a serving bowl and serve.