Rinse and wipe excess moisture from 3 large tomatoes and 1 large onion. Chop them roughly into large pieces.
Heat 2 tbsp oil in a pan, add tomato and onion. Saute well and cook for 6-8 minutes in medium flame. Saute and cook well until the tomatoes wilt and turn soft. Remove from flame. Let it cool
Transfer the sautéed onion and tomato mixture in to a mixer jar.
Add kashmiri chilli powder and salt as required. Grind to a smooth paste. Add little water if needed but do not add more than 2-3 tablespoons.
Heat 2 teaspoons oil in a pan. Add 1 teaspoon mustard seeds and few curry leaves.
Pour the ground chutney into it and mix well.
Serve onion tomato chutney with breakfast like idli or dosa