Put the lentils and measured water into a heavy pot and bring to the boil. Remove any scum that collects at the top. Add the turmeric and stir to mix. Cover, leaving the lid very slightly ajar, reduce the heat to low and simmer gently for 45–60 minutes. Stir a few times during the last half hour.
While the lentils cook, heat the oil in a 20–23cm/8–9 inch frying