Preheat the slow cooker if necessary; see the manufacturer's instructions. Heat the oil in a large frying pan, add the onion and fry, stirring, for 5 minutes or until softened. Add the curry sauce and bring to the boil.
Put the cauliflower and lentils into the slow cooker pot, then pour over the hot sauce. Cover with the lid and cook on low for 5–6 hours or until the cauliflower is tender.
Stir the cauliflower and lentil mixture and sprinkle the spinach on top. Replace the lid and cook, still on low, for 10–15 minutes or until the spinach has just wilted. Spoon into bowls and serve with warm naan bread.