Rinse the toor dal under cold water until water runs clear, then soak for 15 minutes.
In a pressure cooker or heavy-bottomed pot, combine soaked dal, chopped tomatoes, slit green chili, turmeric powder, salt, and 3-4 cups water. Cook for 20-25 minutes until dal becomes mushy and well-cooked.
Heat oil or ghee in a small pan over medium heat. Add mustard seeds and let them splutter.
Add cumin seeds, fenugreek seeds, and dry red chili to the pan. Sauté for 30 seconds until fragrant.
Add curry leaves (they will splutter), followed by chili powder and coriander powder. Quickly add any remaining chopped tomatoes and sauté briefly.
Pour the entire tempering mixture into the cooked dal and stir well.
Simmer the dal for 5 minutes, adjusting consistency with water if needed.
Taste and adjust salt, then serve hot with steamed rice.