Heat the bacon drippings in a large heavy pot over medium heat until melted and shimmering, about 1 minute.
Add the onion (finely chopped) and cook, stirring, until translucent, 5–7 minutes. Add the garlic (minced) and red pepper flakes and cook until fragrant, 30–60 seconds.
Pour in the chicken broth and add the smoked ham hock. Bring to a boil, then reduce to a gentle simmer, cover, and cook 45–60 minutes, until the meat is nearly tender and the liquid is well flavored.
Meanwhile, prepare the greens by removing tough stems and slicing the leaves into 1-inch strips.
Uncover the pot; add half the collard greens, stirring until wilted, then add the rest. Season with the kosher salt and black pepper. Cover and simmer gently 45–60 minutes, stirring occasionally, until the greens are very tender and silky and the pot liquor tastes rich.
Stir in the apple cider vinegar and granulated sugar; simmer 5 minutes. Lift out the ham hock, shred the meat from the bone, and return it to the pot; discard skin and bone. Taste and adjust seasoning and vinegar as needed. Serve hot with hot pepper vinegar.