Chop onions, tomatoes and spring onions separately and keep aside. Heat oil in a pan, once it is hot; add mustard and cumin seeds (jeera). Once they start to splutter add urad dal, chopped onions, turmeric powder, curry leaves and fry till the onions are translucent then add the chopped tomatoes and mix well. Now add chilli powder, salt to taste, chopped spring onions and mix well. Cover with lid and cook till the tomatoes are soft (you need not add any water as tomatoes gives required water for the gravy but if the curry is too thick then add required quantity of water) and cook until you get a thick consistency. Once done, remove lid and add garam masala powder, mix well and serve.