Prick the sausages. Fry or grill them until cooked and brown. Add 1 or 2 pineapple rings and heat through with the sausages for a minute or two. Save 2 sausages, and keep hot with the pineapple rings (for decoration). Slice the rest of the sausages.
Peel and slice the onion, or wash and chop the spring onions. Chop another 1 or 2 pineapple rings.
Heat the butter in a saucepan over a moderate heat, and fry the onion and pineapple pieces until soft (2-3 minutes). Add the beans, sliced sausages, vinegar, salt and pepper, and simmer for 3-4 minutes.
Pour into a warm serving dish. Decorate with the whole sausages and pineapple rings you are keeping hot. Serve with crispy bread rolls or hot toast.