1. Top and tail the beans and pull off any hard strings around the edges; wash in cold water and drain.
2. Place into boiling salted water, bring back to the boil, skim and boil uncovered fairly rapidly for 4 minutes, keeping the beans crisp and green.
3. Drain, reserving the cooking liquor, then thicken 500 ml of the cooking liquor with 25 g arrowroot, add 25 ml vinegar and cook for 10 minutes to form a light coating sauce. Strain over the beans and serve.