Heat oil. Add urad dal, mustard seeds and cumin seeds. Let it splutter.
Add chopped onion, garlic and dried red chilies. Sauté on medium heat till onion is translucent. Add asafetida. Mix. Turn the heat off and cook the onion mixture.
Add the onion mixture to a food processor. Blitz. Add the roasted peanuts, half a cup of water, salt, black pepper powder and tamarind powder. Make into a smooth paste.
Pour mixture in a bowl and serve with idli and sambar.