Cook bacon strips in a large nonstick skillet over medium-high heat, stirring occasionally, until crisp and browned, about 7 minutes. Transfer the bacon to a paper towel–lined plate, reserving the drippings in the skillet.
Add trimmed green beans and ½ cup onion to the drippings; cook over medium-high heat, stirring occasionally, until the beans are blistered, about 3 minutes. Reduce heat to medium; add 2 tablespoons water. Cover and cook, undisturbed, until the beans are tender and the water has mostly evaporated, about 5 minutes.
Meanwhile, in a small bowl, stir together 2½ tablespoons vinegar, 1 tablespoon brown sugar, 1 teaspoon mustard and ⅛ teaspoon salt. Add the vinegar mixture to the beans; cook, stirring constantly, until the sugar has dissolved and the beans are coated, about 2 minutes. Top with the bacon.
