Peel the potatoes and cut them into chunks measuring about an inch. Add them to the slow cooker (mine's a 3.5 litre one) along with the sliced onion, water, crumbled stock cube, oil, spices, and plenty of black pepper. Top with a couple of big handfuls of fresh spinach - however much you can easily fit in your slow cooker. I couldn't fit all of the spinach in, so I came back and added the rest about an hour into the cooking time, once it had wilted down and created some space.
Cook on medium (or high if you don't have a medium setting) for around 3 hours, until the potato is soft - stir every hour or so to scrape down the sides. The exact cooking time will depend on exactly how big you cut your potato chunks.
Recipe by Amuse Your Bouche at http://www.amuse-your-bouche.com/easy-slow-cooker-saag-aloo/