Heat the olive oil in a large pot or Dutch oven over medium heat. Once hot, add the chopped onion and sauté until translucent, about 5-7 minutes.
Add the minced garlic to the pot and cook for an additional minute, stirring constantly to avoid burning.
Gradually add the chopped collard greens to the pot, stirring to incorporate with the onions and garlic. If necessary, do this in batches to manage the volume of greens.
Pour in the vegetable or chicken broth, and stir in the apple cider vinegar, red pepper flakes, salt, and black pepper.
If using, add the smoked turkey leg to the pot for extra flavor. Bring the mixture to a boil, then reduce to a simmer.
Cover the pot and allow the collard greens to simmer for about 30-40 minutes, or until they are tender. Stir occasionally and adjust seasonings to taste.
If desired, sprinkle in sugar to balance the bitterness of the greens. Simmer for another 5 minutes before serving.
Remove the smoked turkey leg, if used, and shred any meat back into the greens (or discard). Serve hot alongside your favorite soul food dishes.