Method. Heat oil in a large non-stick frying pan over medium heat. Add onion, carrot and celery. Cook, stirring often, for 5 minutes or until softened. Add garlic and ginger. Cook for 1 minute or until fragrant. Add cumin and coriander. Cook, stirring, for 30 seconds or until fragrant. Transfer to the bowl of a 5.5 litre slow cooker. Add lentils, stock and 2 cups cold water. Season with salt and pepper. Cover with lid. Cook on low for 4 hours or until thickened. Divide between serving bowls. Top with coriander and yoghurt. Serve with naan bread.