First wash the tomatoes and chop them to quarters along with the onions and keep aside.
Heat a pan in medium flame and add 1 tsp oil, add the urad dal, allow it to slightly brown, add in the red chilli, onion and some salt. Cook until the onion is slightly translucent.
Next add the chopped tomatoes and ginger to the onions and cover and cook until the tomatoes go soft.
Add a piece for tamarind, switch off the flame. Allow the mixture to cool down.
Transfer the cooked chutney mixture to a blender, add a little bit of water and blend it to a smooth paste.
Transfer to a serving bowl
In a small pan add oil, once its hot add mustard seeds, allow it to crackle, reduce the flame to low
Add urad dal, curry leaves and switch off the flame. Allow the ingredients to sit in the mixture for about 1 minute. This tempering is called tadka.
Pour the tadka over the prepared chutney and serve it with dosa or idli or pesarettu