Rinse the collard greens thoroughly to remove any grit. Drain and set aside.
In a large pot or Dutch oven, heat the olive oil over medium heat.
Add the diced onion and sauté for about 5 minutes, or until the onions become translucent.
Stir in the minced garlic and cook for an additional minute until fragrant.
If using, add the smoked turkey leg or ham hock to the pot.
Pour the chicken or vegetable broth into the pot and bring to a boil.
Once boiling, add the collard greens to the pot. Stir well to combine.
Reduce the heat to low, cover, and let simmer for about 45 minutes, stirring occasionally.
After 45 minutes, add the apple cider vinegar, red pepper flakes (if using), salt, and black pepper to taste. Stir well.
Remove the turkey leg or ham hock from the pot, if used, and let cool slightly before shredding the meat into small pieces, discarding the bone and skin.
Return any shredded meat back into the pot, if desired. Taste and adjust seasoning as needed.
Serve hot as a side dish.