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 lightly browned.
Mix in the cumin seeds, cardamom pods and seeds, onion seeds (if used), ground spices and ginger. Cook for 1 minute, then mix in the tomatoes, stock, sugar and season with salt and pepper. Bring to the boil, stirring.
Cut the potatoes into thick slices or halves (if they are small) so that all the pieces are of a similar size. Transfer to the slow cooker pot and pour the sauce over the top. Cover and cook on high for 6–7 hours or until the potatoes are tender.
Add the spinach and cook, still on high, for 15 minutes more until it is just wilted. Stir the curry and serve sprinkled with torn coriander leaves and accompanied with warm naan bread and a lentil dhal and plain rice, if liked.