1. Preheat a grill over medium-high heat. Lay 2 sheets of aluminum foil, about 14 inches long, on top of each other and crimp the edges together and up slightly so it resembles a rimmed baking sheet. Put the beans in the center and toss with 1 tablespoon of the oil, 1/4 teaspoon salt and 1/4 teaspoon pepper.
2. Place the foil baking sheet on the grill and add 2 tablespoons water. Grill the beans, tossing with long tongs occasionally, until lightly charred and softened, about 10 minutes. Add the onions and another 2 tablespoons of water and continue to grill until the beans and onions are tender, another 6 to 7 minutes.
3. While the beans grill, cook the bacon in a small skillet over medium heat until crispy; transfer to a paper-towel-lined plate to drain. Pour off the bacon drippings, leaving about a tablespoon in the skillet. Remove the skillet from the heat and immediately stir in the mustard, vinegar and remaining 1 tablespoon oil until combined.
4. Transfer the beans and onions to a bowl and add the vinaigrette from the skillet. Toss well, crumble the bacon over top and serve.