French beans are equally good served hot or at room temperature, and either way they can be cooked in advance. You do, however, need to keep a close eye on them: undercook and they will be too crunchy, cold in the middle and slightly raw-tasting; overcook and they are likely to be waterlogged and taste rather dull. The idea is to boil them to exactly the right point and then immediately refresh them under cold water – both to stop the cooking process and to retain their vivid green colour.
If serving the beans at room temperature, as here, dress them just beforehand. If serving hot, gently warm them in a pan with a little olive oil, a pinch of salt and plenty of pepper, sprinkling over a finely chopped garlic clove if you like.