
Heat the oil in a saucepan and soften the onion and garlic for 2 to 3 minutes.
Add the tomatoes and cook until they break down into a soft base, about 5 minutes.
Loosen the groundnut paste with a splash of water until smooth, then stir it into the pot.
Add the sweet potato and the 3 cups of water. Bring to a gentle simmer.
Cover and cook for 20 to 25 minutes, stirring occasionally, until the sweet potato is tender and the sauce has thickened.
Stir in the spinach and cook for 2 to 3 minutes until just wilted.
Season with salt and serve hot.