Melt the butter in a pan with the olive oil. Add the onions and a pinch of salt and fry for 25-30 mins or until caramelised. Add the cider and simmer until reduced. Stir in the sugar and vinegar. Mix the mustard with the mayonnaise. Cook the sausages following pack instructions. Split the hot dog buns and fill each with a sausage. Top with the onions and the mustard-mayo.