Heat oven to 325°F.
In a small bowl stir together the salt, cumin, black pepper, paprika and cinnamon. Season the oxtails all over with the spices. Heat a large Dutch oven or heavy bottomed saucepan over medium heat, add the oil and heat through.
Working in batches if needed, arrange the oxtails in a single layer in the cast iron skillet. Sear the oxtails golden brown on all sides, about 6 minutes total. Transfer meat to a plate, repeating until all oxtails are browned.
Return the pot to medium heat, add the carrots, celery and onion and sauté, stirring occasionally until the vegetables are tender, about 6 minutes. Add the garlic and cook, stirring frequently, until fragrant, about 30 seconds.
Add the wine to the Dutch oven and simmer until it’s almost completely evaporated, about 8 to 10 minutes. Pour in the beef stock followed by the thyme and red pepper. Arrange the oxtails in the pot and bring to a simmer. Cover the pot and transfer to the oven.
Cook, turning oxtails half-way through cooking, until the meat is tender, about 3 hours. Remove the oxtails from the pot and let cool. Once cool, pull the oxtail meat from bones and shred the meat, discarding any large pieces of fat.
Add the meat back to the pot with the mustard greens and stir to combine, cooking over medium-low heat until the greens wilt, about an additional 10 minutes.