Preheat your oven to 300°F (150°C).

In a large Dutch oven, heat the vegetable oil over medium-high heat. Season the oxtails with salt and pepper, then brown them in batches, about 3-4 minutes per side. Remove the oxtails and set aside.

In the same pot, add the chopped onion, garlic, carrots, and celery. Sauté for about 5-7 minutes until the vegetables are softened.

Stir in the thyme, rosemary, and bay leaf, cooking for another minute until fragrant.

Pour in the red wine, scraping up any browned bits from the bottom of the pot. Let it simmer for about 5 minutes.

Add the beef broth and tomato paste, stirring to combine. Return the oxtails to the pot, ensuring they are submerged in the liquid.

Cover the pot and transfer it to the preheated oven. Braise for 3-4 hours, or until the meat is tender and falling off the bone.

Once done, remove the pot from the oven and let it sit for 10 minutes. Skim off any excess fat from the surface.

Serve the oxtails hot, garnished with fresh parsley.