In a large Dutch oven or heavy-bottomed pot, cook the chopped bacon over medium heat until it's crispy and has rendered its fat, about 8-10 minutes. The bacon should be evenly browned but not burnt.
Using a slotted spoon, transfer about half of the bacon to a paper towel-lined plate for garnish, leaving the remaining bacon and all of the bacon fat in the pot.
Add the diced onion to the pot with the bacon fat and cook until translucent, about 5 minutes.
Add the minced garlic and cook for another 30 seconds until fragrant but not browned.
Add the trimmed and snapped green beans to the pot, stirring to coat them in the bacon fat mixture.
Pour in the chicken broth, apple cider vinegar, sugar, and red pepper flakes (if using). The liquid should come about halfway up the beans – not completely covering them.
Bring the mixture to a boil, then reduce heat to low. Cover and simmer for 30-45 minutes, or until the green beans reach your preferred tenderness.
Once the beans have reached your preferred tenderness, season with salt and pepper to taste.
Transfer to a serving dish and top with the reserved crispy bacon pieces.
Allow the beans to rest for 5 minutes before serving to allow the flavors to settle and intensify.