Heat the oil in a saucepan over a medium heat, add the onion and sauté for 5 minutes or until soft. Add the spices and salt and cook, stirring, for 3–4 minutes, then add the tomato paste and cook for a further 2 minutes. Add the sugar and vinegar, increase the heat and cook until the liquid has been reduced to a glaze, then add the tinned tomatoes and 200 ml water and bring to the boil. Cook for 8–12 minutes until the sauce has a nice, thick consistency, then add the tinned beans and warm through. Leave to sit for 5 minutes before serving.