Heat the sunflower oil in your potjie pot (or Dutch oven) over medium heat.
Add in the oxtail pieces and brown them on all sides. You may need to do this in batches to ensure each piece gets a nice sear. Once browned, remove the oxtail from the pot and set aside.
In the same pot, add the onions and garlic, sautéing them until they become soft and fragrant.
Return the browned oxtail to the pot, then add in the beef stock, red wine, Worcestershire sauce, bay leaves, and dried thyme.
Cover the pot and reduce the heat to low. Allow the oxtail to simmer for about 3 hours, or until the meat is tender and falling off the bone. Make sure to stir occasionally to prevent the oxtail from sticking to the bottom of the pot.
After 3 hours, add in the carrots, potatoes, and green beans. Season with salt and pepper, then give everything a good stir.
Cover the pot again and let it simmer for another hour, or until the vegetables are tender.
Adjust the seasoning if necessary, then serve your oxtail potjie piping hot, ideally with a side of steamed rice or fresh bread.