Preheat the slow cooker if necessary; see the manufacturer's instructions. Heat 1 tablespoon of the oil in a large frying pan, add one-third of the aubergines and fry on both sides until softened and golden. Scoop out of the pan with a slotted spoon and transfer to a plate. Repeat with the rest of the aubergines using 2 more tablespoons of oil.
Add the remaining oil to the pan and fry the onion for 5 minutes or until softened. Stir in the garlic, paprika and cumin and cook for 1 minute, then mix in the lentils and tomatoes. Add a little salt and pepper and bring to the boil. Pour the mixture into the slow cooker pot and stir in the boiling stock.
Cover with the lid and cook on low for 6–8 hours. Serve the soup as it is or purée it with a stick blender, if preferred. Ladle the soup into bowls, drizzle with a little extra olive oil and sprinkle with coriander. Serve with toasted ciabatta bread.