In a large pot, heat olive oil over medium heat. Add onion and garlic, sauté until soft.
Add the smoked turkey and pour in the broth. Bring to a boil, then reduce to a simmer for 10 minutes.
Add chopped collard greens, black pepper, and red pepper flakes. Stir well.
Cover and let simmer for about 45 minutes to 1 hour, stirring occasionally, until greens are tender.
Remove the turkey leg, shred the meat, and stir it back into the greens.
Add apple cider vinegar, adjust seasoning, and serve warm.