Peel and finely chop the onion. Peel and dice the sweet potato, then finely chop the green beans. Heat the oil in a large pan on a medium heat, add the onion, then fry for 5 minutes, or until softened, stirring regularly. Drain and add the kidney beans, along with the sweet potato and tomato purée. Pour in 200ml water, then simmer with the lid on for 15 minutes. Add the green beans and cook for a further 5 minutes, or until the sweet potato and green beans are tender. Stir in the peanut butter. Mash with a fork to make a fairly smooth mixture with some soft lumps, then leave to cool. For more information on stages of complementary feeding, click here.