These yellow runner beans are often seen in Italian markets. When beans look like this there is very little you need to do to them. If you can't find them, then use runners or normal green beans.
200-250g yellow, flat, or runner beans
For the dressing
2 medium red onions, finely chopped
100ml good-quality red wine vinegar
4-5tbsp olive oil
Salt and freshly ground black pepper
Put the red onions in a saucepan with the vinegar and 2tbsp water. Bring to the boil; simmer for 3-4 minutes until the liquid has almost evaporated.
Transfer to a bowl, whisk in the oil; season.
Cook the beans in boiling, salted water for 2-3 minutes until tender then drain; arrange on serving plates and spoon the dressing over.