In a large deep skillet, cook the bacon over moderate heat, stirring, until golden. Transfer the bacon with a slotted spoon to a plate. Add the onion to the skillet and cook, stirring, until softened. Add the beans, pepper flakes, and salt and saute over moderately high heat, stirring, for 2 minutes.

Add the boiling water and cover the skillet immediately. Steam the beans, shaking the skillet occasionally, for 15 minutes, or until just tender. Add the butter, vinegar, and salt and pepper and toss until combined. Sprinkle with the bacon.