Heat the oil in a heavy-based saucepan, that has a lid. Peel and slice the onion, and add to the oil. Cook for approximately 4 minutes over a medium heat.
While the onion is cooking, peel and chop the sweet potatoes into roughly 2cm cubes.
Add the garlic, ginger, cumin, turmeric and chilli (if using) to the onion. Stir in the sweet potato, and mix it with the spices and onions. Tip in the tinned tomatoes and water. Cover, and bring to a gentle simmer for 20 minutes.
After 20 minutes, test the potatoes are tender and add in the chickpeas and spinach. Wilt for a minute and ensure the chickpeas are heated through. Serve immediately.