Add all ingredients in a saucepan (except corn starch). Simmer on medium heat for 5 minutes till jaggery melts into the sauce.
Add 1 tbsp corn starch with 1 tbsp water. Mix well. Add this mixture to the boiling sauce and simmer till desired consistency arrives. Don't make very thick as after cooling the corn starch will also give thicker consistency to the sauce.
Keep stirring frequently to avoid burning at the bottom. If it thickens more add a little extra water.
Remove from heat and taste the spicy, sour and sweetness of the sauce. Add chillies and jaggery if required and give it a boil.