Heat ½ tsp of oil in a pan or kadai. Add the urad dal, chana dal, and dried red chilies broken into small pieces when the oil is hot.
Roast the lentils until they turn nice golden brown like below. Transfer it to the mixer jar and let it cool.
In the same kadai/pan, add 1 tsp of oil. When the oil is hot, add the chopped onion and garlic.
Cook until the onions turn soft and translucent. It took me approx 7 minutes. Transfer it to the mixer jar.
In the same kadai, add the chopped tomatoes.
Cook till the tomatoes are slightly soft and mushy. Let it cool.
First, grind the lentils and the onions coarsely.
Now add the tomatoes and salt and grind it smooth. No need add water and if required add 1 to 2 tbsp.
In a separate tempering pan, heat oil. Add the mustard seeds, urad dal, and asafoetida when the oil is hot. Add the curry leaves when the mustard seeds splutter and turn off the heat. Let the curry leaves crisp up in the residual heat. Add this tempering to the chutney. Drizzle the gingelly oil (if using) and serve it with idli and dosa.
Notes
I used groundnut oil for sauteing and tempering. You can use gingelly oil or oil of your choice.
You can add a small piece of tamarind or ¼ tsp of tamarind paste for a tangy taste.
Adjust salt and red chilies to taste. For a bright red color chutney, use byadgi or Kashmiri red chilies or a combination of Guntur/byadgi or Guntur and Kashmiri red chilies.
You can add up to 10 grams of garlic, 4 to 5 cloves.