Pat the oxtail pieces dry with paper towels and season them evenly with salt, black pepper, smoked paprika, garlic powder, and onion powder. Heat the olive oil in a large skillet over medium-high heat and sear the oxtail on all sides until deeply browned, about 2–3 minutes per side. Browning develops a rich, caramelized flavor that makes the finished gravy even more delicious.

Add the chopped onion, carrots, celery, and minced garlic to the same skillet. Cook for about 5 minutes, stirring occasionally, until the vegetables soften and become fragrant. Stir in the tomato paste and cook for another minute to deepen its flavor.

Transfer the browned oxtail and sautéed vegetables to the slow cooker. Pour in the beef broth and Worcestershire sauce, then add the thyme, rosemary, and bay leaves. Stir gently so the ingredients are evenly distributed.

Cover the slow cooker with the lid and cook on Low for 8 hours or High for 5–6 hours. The meat is ready when it becomes extremely tender and easily pulls away from the bone with a fork.

Carefully remove the bay leaves. Mix the cornstarch and cold water in a small bowl to create a slurry. Stir the slurry into the slow cooker and cook on High for another 15–20 minutes, or until the gravy thickens into a rich, glossy sauce.

Once the gravy has thickened, gently stir the oxtail to coat each piece in the rich sauce. Remove the bay leaves if you haven’t already. Sprinkle freshly chopped parsley over the top for a pop of color and freshness. Serve the slow cooker oxtail hot with your favorite side dishes, making sure to spoon plenty of the savory gravy over each serving for the ultimate comfort meal.