Rinse dal under running water; drain. Place dal, water and turmeric in a medium pot, and bring to the boil. Cover, reduce heat, and cook until dal is tender, about 30 minutes.
Meanwhile, place cubed sweet potato in a steamer over 2.5cm (1 in) of boiling water. Cover and cook until tender but still firm, about 15 minutes (time may vary depending on size of the pieces). Once tender, mash sweet potato and dal together; set aside.
Heat the oil in a frying pan over medium heat, and cook the onion until lightly browned. Stir in tomato, chillies, salt and pepper, and continue cooking another 3 to 5 minutes. Place lentils, vegetables and potatoes in a stockpot over medium heat. Stir in water as needed to attain desired consistency. Serve warm with coriander or spring onions.
I use Toor dal, which is the most widely used dal in the country. You could also substitute chane ki dal (yellow split peas) in a pinch.