Heat a pan. Add the mustard and cumin seeds and wait for them to sputter and crackle. Add the tomatoes, green chilli and cook for a bit. Add dal, turmeric powder and some water and bring to a boil. Add salt and garnish with chopped coriander and lime juice.
Variation: You could avoid tomatoes. Add other flavours like ginger, garlic, fenugreek seeds, etc., as per taste.