Heat the oil in a large frying pan, add the onions and red pepper and cook over a medium heat for 3 minutes until starting to soften. Add the tomatoes and garlic and cook for a further 2 minutes, then add the curry powder, turmeric and lentils and stir well.
Pour in the stock and bring to the boil. Reduce the heat, cover and simmer gently for 15 minutes until the lentils are completely cooked through and soft, adding a little more water if necessary. Season well.
Meanwhile, heat the naans in a preheated oven, 180°C (350°F), Gas Mark 4, for 5–8 minutes, or until warm.
Spoon the dahl on to the warm naans, scatter with chopped coriander and serve with spoonfuls of yogurt.