Squeeze the sausagemeat from one of the sausages, then roll into four balls. Repeat with the rest of the sausages. Heat half the oil in a flameproof casserole, shallow saucepan or deep frying pan over a medium heat and fry the sausage balls for about 8 mins, stirring occasionally until browned all over.
Scatter in the onion, drizzle in the remaining oil and cook for 5 mins until the onions are golden. Add the garlic and cook for 2 mins more. Scatter in 1 tbsp sugar and the fajita spice mix, cooking for a minute or two until the mixture has thickened. Splash in the vinegar, tip in the tomatoes and a can of water, then add the beans. Season, stir, bring to a simmer and bubble gently for 15-20 mins until the sauce is thick and rich. Remove from the heat and leave to cool slightly. Can be prepared up to three days ahead, then cooled completely and chilled for up to three days or frozen for three months. Defrost completely in the fridge overnight and reheat in a pan until piping hot. Scatter with parsley or coriander and serve with crusty bread or jacket potatoes, if you like.