Wash and trim the beans; pat dry.
Cook the chopped bacon in a large skillet over medium heat until crisp, 6–8 min; remove to a paper-towel-lined plate; keep 2 Tbsp bacon grease in the pan.
Add butter + onion (if using) to the grease, cook 2 min; add garlic, cook 30–60 sec until fragrant — do not brown.
Add the beans; toss in the bacon fat and sauté 6–8 min until bright green and tender-crisp.
Add brown sugar + soy sauce (+ Worcestershire if using); stir until the butter melts and the sugar dissolves into a glossy glaze, 2–3 min.
Return the crispy bacon; toss, cook 1–2 min more; season with salt + pepper + red pepper flakes if using. Serve immediately.