Trim and snap the green beans into 2-inch pieces. Slice the onion and mince the garlic so everything is ready to go.
Place a heavy pot or Dutch oven over medium heat. Add the chopped bacon and onion; cook, stirring often, until the bacon renders and begins to crisp and the onion softens and turns light golden, 7–8 minutes.
Scoop out about half of the bacon with a slotted spoon and reserve for finishing. Leave the remaining bacon, onions, and drippings in the pot.
Stir in the minced garlic and cook just until fragrant, about 30 seconds. Splash in 2 tablespoons of the broth and scrape up any browned bits from the bottom of the pot.
Add the green beans and toss to coat in the drippings for 1–2 minutes until glossy and bright in spots.
Pour in the remaining broth. Season with the salt and black pepper. Bring to a gentle boil, then reduce heat to low, cover with the lid slightly ajar, and simmer for 35 minutes, stirring every 10 minutes.
Uncover and continue cooking 10–15 minutes more, allowing the liquid to reduce slightly and the beans to become very tender. If the pot looks dry before the beans are done, add a splash of broth or water.
Finish with the apple cider vinegar. Taste and adjust salt and pepper as needed. Serve hot with the reserved crisp bacon sprinkled over the top.