Heat oil in a pan and add hing, garlic and onions. Add salt and saute till their raw smell disappears and onions turn transparent.
Then add tomatoes and cook until tomatoes become soft and mushy.
Remove from flame and allow the mixture to cool. Once it cools, grind to a smooth paste.
Heat 1 teaspoon oil and add dry red chilli and mustard seeds. When seeds splutter, add urad dal and curry leaves, sauté for a few seconds and add the prepared onion-tomato paste. Check the salt level and adjust accordingly.
Mix well and cook until oil separates. Tomato-onion chutney is ready. Serve with idli, dosa, parathas, upma or any snack.