Add the bacon to a large skillet and cook over medium heat, stirring occasionally, until brown and crisp, about 6 minutes. While the bacon cooks, cut the stems from the bok choy into 1-inch pieces and the leafy tops into 3-inch pieces.
Add the onion to the bacon and cook, stirring, until softened slightly, 4 to 5 minutes. Toss in the beans and stir until heated through, about 2 minutes. Transfer the vegetables with a slotted spoon to a bowl, leaving the fat in the pan.
Add the bok choy stems to the pan, increase the heat to medium-high and cook, stirring, until slightly softened, about 2 minutes. Add the bok choy leaves and cook, stirring, until wilted, about 1 minute. Remove the pan from the heat and stir in the vinegar. Return the onion-bean mixture back to the pan and season with salt. Serve warm or at room temperature.
From Food Network Kitchens