Heat the oil in a saucepan. Add the mustard and when it sputters, add the onions and saute for a few minutes until the onions turn translucent.
Add the ginger and garlic and saute another minute.
Add the coriander powder, chilli powder and turmeric and stir to coat with oil.
Add the tomatoes and beet greens. Cook until the beet greens have wilted and the tomatoes are breaking down.
Add the cooked mung dal and stir together, adding some water if the curry is too thick. Add salt to taste.