Top and tail French beans and string and slice runner beans. Cook in boiling salted water for 10–15 minutes or until tender, then drain well.
Put the butter in the hot rinsed-out pan and add the finely chopped onion. Fry for a few minutes until the onion is softened and beginning to brown. Return the beans to die pan and toss to blend with the butter and onion. When hot and glazed serve at once.