Bring a large pot of salted water to a boil. Add the green beans and cook until tender, about 7 minutes. Drain, rinse under cold water and blot dry with paper towels; set aside. (This can be done up to 4 hours ahead; cover and refrigerate.)
Heat the olive oil in a large pot or Dutch oven over medium-high heat. Add the onion, season with salt and cook, stirring occasionally, until soft and golden, about 8 minutes. Add the ham and continue cooking until warmed through, about 2 minutes. Stir in the vinegar and remove from the heat.
Add the green beans to the pot and stir until combined and warmed through. Add the parsley and season with salt and pepper.