In a large pot at medium high heat, cook bacon for 8-10 minutes or until it is just about to get crispy. Add chopped onion and minced garlic, cook for 3-4 minutes more.
Add salt, pepper and red pepper flakes for seasoning then add broth, apple cider vinegar, and collard greens. Stir and continue to cook on medium high until a slow boil is achieved, then reduce heat and simmer for 2 hours. Stir occasionally and cook until the collard greens are nice and tender (or to your preferred tenderness).