In a kadai/Pan add in a tsp of oil, once the oil becomes hot add in finely chopped onions sauté until the raw smell of the onions leaves or till the onions turn glossy. Next, add in finely chopped tomatoes.
Sauté the onions for about 3 mins and then add in salt and chili powder and mix everything really well and then cook for about 1 min and then switch off the stove. Allow the tomato-onion mixture to cool down.
Once the mixture cools down take it in a blender and blend it into a fine paste. Transfer it into the serving bowl now in another Kadai add in the remaining 1 tsp of oil once it heats up add in mustard seeds once it splutters add in curry leaves and switch off the flame. Pour the tadka over the ground tomato chutney. Tasty tomato chutney is now ready.
Notes
Using gingelly oil for the chutney gives a nice aroma to the chutney.
You can also replace the chili powder by adding two red chilies. If you are adding red chilies you can add them before adding the onions.
Pairs well with Idli, Dosa, Chapathi even with Curd Rice.