Heat the olive oil or bacon grease in a large skillet over medium-high heat. If using bacon, add the pieces now and cook until slightly crisp; drain off excess fat, leaving about 1 tablespoon in the pan. If using ham hock, skip the bacon step.

Add the chopped onion to the skillet and cook for 5 to 7 minutes until softened. Add the minced garlic, red pepper flakes, and dried thyme. Cook for 1 minute until fragrant.

Transfer the sautéed onions, garlic, and bacon (if used) to the bottom of the slow cooker. If using a ham hock or turkey leg, place it directly on top of the aromatics.

Add the rinsed black eyed peas to the slow cooker, spreading them over the meat and aromatics. Pour in the 6 cups of broth and 2 cups of water. The liquid should cover the peas by at least 1 inch; add more if necessary. Add the bay leaf, 1/2 teaspoon of the kosher salt, and the black pepper.

Place the chopped collard greens directly on top of the peas and liquid. They will look overflowing, but they will cook down significantly. Do not stir yet.

Cover the slow cooker and cook on LOW for 7 to 8 hours or on HIGH for 3.5 to 4 hours. The peas are done when they are very tender and creamy, and the greens have wilted completely and are soft. If cooking on LOW, you may gently stir the pot after about 4 hours to ensure the greens are submerged.

Once the peas are tender, use tongs to carefully remove the ham hock or turkey leg from the pot. Place it on a cutting board, shred the meat, discarding the bone and any excess skin or fat. Return the shredded meat to the slow cooker.

Taste the liquid and adjust the seasoning. Stir in the remaining 1/2 teaspoon of salt, more pepper, or additional red pepper flakes, if desired. Stir in the 1 tablespoon of apple cider vinegar to brighten the flavor.

Allow the mixture to rest, uncovered, for 10 to 15 minutes before serving to thicken slightly. Serve hot with the pot liquor (the flavorful broth) over rice or cornbread.