Cook the vaal dal in a pressure cooker or on the stovetop in a saucepan. To cook in a saucepan, cover the dal with 1 inch of water, bring to a boil, turn the heat to a simmer, slap on a lid, and cook about 20-30 minutes until the dal is very tender. Add more water if the dal starts to get too dry.
Heat the oil in another saucepan. Add the mustard seeds and when they sputter, add the curry leaves and onions.
Saute until the onions are transparent, about five minutes, then add the ginger and garlic.
Stir fry for half a minute or until the garlic becomes lightly blond. Add the sweet potatoes and the sambar powder and stir well to coat the sweet potato with the spices. Cook, stirring, another five minutes or until the sweet potatoes start to soften.
Add the cooked vaal dal and mix well. Bring the curry to a boil, lower the heat, put on a lid, and let the dal simmer away another 10 minutes or until the sweet potatoes are completely tender. Add water if the dal gets too dry. Vaal dal will turn really soft and break up when it's well-cooked, which is great because that's what gives this curry its really creamy consistency without any cream.
Add salt to taste and mix in the coriander. Turn off the heat and add the lemon juice.
Serve hot with some rice or rotis.