Preheat the slow cooker if necessary; see the manufacturer's instructions. Rinse the lentils well with cold water, drain and put into the slow cooker pot with the onion, spices, ginger, tomatoes and boiling stock.
Stir in a little salt and pepper, cover with the lid and cook on high for 3–4 hours or until the lentils are soft and tender.
When almost ready to serve, make the tarka. Heat the oil in a small frying pan, add the remaining tarka ingredients and fry, stirring, for 2 minutes. Roughly mash the lentil mixture, then spoon into bowls, add spoonfuls of yogurt and drizzle with the tarka. Sprinkle with coriander leaves and serve with warm naan bread.