Heat oil in a kadai. When the oil is hot, add urad dal, chana dal, hing ,curry leaves and red chillies .Fry on a medium heat, till dal turns golden. Take out on a plate to cool
Transfer the fried masala, grated sweet potato ,coconut ,tamarind and salt to a mixer / blender. Grind to a smooth paste...but don't make super smooth. Taste test .
Prepare the tempering .Heat oil in a small pan. When oil is hot, add mustard and curry leaves. Now pour this tempering on grounded chutney .
Serve sweet potato chutney with hot rice / idli / dosa / chapathi etc . Stays good for 3-4 days in fridge .
Notes
If you don't like hing , you can flavor the chutney with little onion / ginger . Add few pieces of it while roasting masala. Fry well and grind. But i would prefer roasting with hing ...