Heat oil in a large non-stick frying pan over medium heat. Add onion carrot and celery and cook, stirring often, for 5 minutes until softened. Add garlic and tomatoes. Cook for 1 minute or until fragrant. Add cumin and ground coriander. Cook for 30 seconds or until fragrant. Transfer to a 5.5 litre slow cooker bowl.
Add lentils, stock and 2 cups of water. Season with salt and pepper. Cover with lid and cook on low for 6-8 hours or until lentils are cooked through.
Serve topped with coriander and yoghurt. Serve with naan bread.