You MUST try this!!!
Soak mustard seeds in vinegar overnight. Then grind to a paste along with ginger and garlic
Heat oil in a large pan until smoking point. Cook slightly. Add cumin, turmeric, chili powder. Stir and fry for few seconds
Add tomatoes and chilies
Pour in the vinegar mixture. Add salt. Reduce heat and simmer until oil begins to separate
Cool and put it in jar