Place bacon in a large skillet and cook over medium-high heat, turning occasionally, until crisp, about 10 minutes.

Meanwhile, place a steamer insert into a saucepan and fill with water to just below the bottom of the steamer. Bring water to a boil. Add green beans, cover, and steam until tender-crisp, 3 to 5 minutes. Drain.

Drain bacon slices on paper towels, reserving grease in the skillet. Saute onion in the bacon grease until softened, about 5 minutes. Stir in brown sugar, vinegar, salt, mustard, and pepper. Stir well, scraping up any onion bits off the bottom of the skillet. Cook until bubbling, 3 to 5 minutes.

Add green beans to the skillet with the sauce; toss to coat well. Chop cooked bacon and stir into the beans. Serve hot.