Wash the collard greens in several changes of water, remove the tough stems, and tear the leaves into large pieces.

In a large heavy pot over medium-high heat, cook the diced onion in oil until softened, about 5 minutes. Add the chopped garlic and red pepper flakes and cook 30 seconds.

Nestle the smoked turkey leg into the pot, then add the chicken broth, apple cider vinegar, sugar, and black pepper. Bring to a boil, cover, reduce to a simmer, and cook for 45 minutes.

Add the torn collard greens in batches, stirring to wilt each addition. Cover and simmer until the greens are tender, 45 to 60 minutes.

Remove the turkey leg, shred the meat from the bone, and discard the skin and bones. Return the meat to the pot, stir, taste, and adjust with salt and vinegar. Ladle into bowls and serve with pepper vinegar or hot sauce.