Heat oil or butter 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.