Set a Dutch oven over medium heat and add the olive oil. Once the oil is hot, add the bacon, stirring occasionally, and begin to sear until most of the fat has rendered and the bacon is well caramelized, about 5 minutes. Add the onions and garlic to the pan and cook, stirring occasionally, until the onions are translucent, about 5 minutes. Add the string beans to the pan and toss well with the bacon and onions. Raise the heat to medium-high and add the chicken stock. As soon as the stock begins to boil, place the lid on the pan and cook the beans for about 6 minutes. Remove the lid, season the beans with the salt and pepper and toss well. Return the lid and cook the beans until tender, about 1 or 2 minutes longer. Remove from the heat and transfer to a serving dish or small platter to serve.