Heat 1 tsp of oil and add the onions.
Stir-fry until the onions start to soften, about 2-3 minutes.
Add the curry leaves, tomatoes and sweet potatoes and stir. Add the ginger, green chillies, and rasam powder and stir together for another 2-3 minutes.
Add the mung dal and stir well. Now add enough water to cover the dal, season with some salt, and pressure cook or let it cook on the stovetop until the dal and vegetables are really soft.
Mash the dal with the back of a ladle. Add water if the mixture is too dry.
Heat the remaining tsp of oil and add the mustard seeds. When they sputter, add the curry leaves. Stir for a few seconds and then add this tempering (or tadka) to the dal mixture.
Add more salt if needed. Serve hot.