Heat the oil in the base of a tagine or a large, heavy-based saucepan over a medium heat, stir in the onions and garlic and cook for 2–3 minutes to soften a little. Add the turmeric, fenugreek and lentils and stir to coat well, then stir in the tomatoes and sugar.
Pour in the measurement water and bring to the boil, then reduce the heat, cover and cook gently for about 30 minutes until the lentils are tender but not mushy, adding a little more water if necessary. Stir in half the coriander and season with salt and pepper. Serve as side dish to grilled or roasted meat and poultry.