Wash your green beans thoroughly under cold water. Trim the ends and snap them into 1-2 inch pieces, keeping the size consistent for even cooking.
In a large, deep skillet or Dutch oven, cook the bacon over medium heat until crispy (approximately 8-10 minutes). Transfer the crispy bacon to paper towels, leaving the rendered fat in the pan.
Reduce heat to medium-low and add the diced onion to the bacon fat. Sauté until translucent, about 5 minutes. Add the minced garlic and cook for another 30 seconds until fragrant.
Add the trimmed green beans to the pot, stirring to coat in the bacon fat. Pour in the chicken broth, season with salt, pepper, and red pepper flakes if using. The liquid should come about 1/3 of the way up the beans.
Bring the mixture to a gentle simmer, then reduce heat to low. Cover and cook for 30-45 minutes, depending on how tender you prefer your beans. Southern-style traditionally means tender but not mushy.
Once the beans have reached your desired tenderness, stir in the apple cider vinegar. Crumble the reserved bacon and add it to the pot. Stir gently to incorporate, then taste and adjust seasonings as needed before serving hot.