Place the lentils into a large container and cover with several centimetres of cold water; let soak 4 hours to overnight. Drain and rinse.
Heat the oil in a heavy-bottomed saucepan over medium heat; cook the onion, green chillies, garlic and ginger in the hot oil until the onions are golden brown, about 5 minutes. Stir the lentils into the onion mixture with 250ml water; season with salt. Reduce heat to low and simmer until the lentils are tender, about 30 minutes. Stir the tomato, garam masala and turmeric into the mixture. Cook until the lentils are cooked through, about 10 minutes more. Garnish with the coriander to serve.