Heat tamarind in 1/3 cup water. When boiling, take off fire. Cool till it can be handled. Put jaggery, tamarind with water and chillies in a small wet grinder. Run till contents are crushed coarsely. Add salt, cumin seeds, run again till smooth. Use as little water as possible. Heat oil, add mustard seeds, allow to splutter. Pour over paste, mix gently till well mixed. Store in sterile bottle in refrigerator till required.