Place the bacon in a large, wide pot and cook over medium heat until some of the fat has rendered and the bacon is light golden, about 5 minutes. Add the onions and cook, stirring occasionally, until softened, about 5 minutes more. Add the mustard greens in batches, letting each batch wilt before adding the next. Add the chicken broth, vinegar, 1/2 teaspoon salt and a few grinds of pepper. Bring to a boil over medium-high heat, then reduce the heat to low and cook, covered, until the greens are very tender, 20 to 25 minutes. Season with salt and pepper and serve with hot sauce on the side.