Put 2 or 3 tablespoons of oil in a large saucepan. Add (i) the chopped onion, (ii) the chopped or grated ginger (if using ground ginger, do not add this yet), (iii) the chopped garlic and (iv) a pinch of salt (see Top Tips). Gently sweat (fry) the mixture for 2 or 3 minutes, stirring to ensure even cooking.
Stir in the ground spices: cumin, turmeric and paprika. If using ground ginger, add this also. Cook the mixture for a further 1 or 2 minutes, until the aroma of the spices is released. Keep to a gentle heat and take care not to burn the spices.
Add the lentils to the spices, stirring to ensure that the lentils are evenly coated in the spice mixture.
Pour the water or stock into the spice mixture. Turn up the heat to bring the mixture to a simmer. Stir occasionally. Simmer until cooked. The diced carrots and lentils should be cooked through at about the same time.
Whilst the soup is simmering, mix together the yoghurt and chopped mint.
Take the soup off the heat and blend to your preferred consistency.
Add the lemon juice to the soup, stirring in thoroughly.
Top each serving of the lentil soup with a dollop of the yoghurt mixture.