Mix the flour, garlic powder, onion powder, paprika, salt, and black pepper in a large bowl. Toss the oxtails in the seasoned flour mixture until evenly coated.

Heat the vegetable oil in a large skillet over medium-high heat. In batches, sear the floured oxtails on all sides until golden brown, about 3-4 minutes per side. Transfer the browned oxtails to the slow cooker.

In the same skillet, add onions, bell pepper, and garlic. Cook over medium heat, stirring occasionally, until softened and fragrant, about 3-4 minutes. Stir in the tomato paste and cook for 1 minute.

Sprinkle the flour over the sautéed veggies and stir well to coat. Gradually whisk in the beef broth, deglazing the skillet and scraping up the browned bits. Stir in the Worcestershire sauce, cayenne pepper, thyme, and bay leaf. Cook for 3-4 minutes, allowing the gravy to thicken slightly.

Pour the gravy over the oxtails in the slow cooker, ensuring the meat is fully submerged. Cover and cook on low heat for 8-10 hours or high heat for 5-6 hours, until the oxtails are fall-off-the-bone tender.

Remove the thyme sprigs and bay leaf from the slow cooker. Taste the gravy and adjust seasoning with additional salt and pepper if needed. Serve the smothered oxtails over white rice, mashed potatoes, or cornbread. Garnish with fresh parsley if desired.