Bring a saucepan of water to the boil. Add the lentils, reduce the heat and simmer for 10 minutes. Drain and refresh under cold running water.
Heat the oil in the base of a tagine or a large, heavy-based saucepan over a medium heat, stir in the onion, garlic, coriander seeds, dried sage leaves and sugar and cook for 2–3 minutes to let the flavours mingle. Add the carrots and cook for a further 1–2 minutes, then add the drained lentils and stir in the tomato purée.
Pour in the measurement water, bring to the boil, then reduce the heat, cover and cook gently for about 20 minutes until most of the liquid has been absorbed. Season with salt and pepper, garnish with the fresh sage leaves and serve with the lemon wedges to squeeze over.