01 - Preheat the oven to 320°F.
02 - Pat the oxtail pieces dry and season generously with salt and black pepper.
03 - Heat olive oil in a large Dutch oven over medium-high heat. Brown oxtail on all sides in batches; set aside.
04 - In the same pot, add onions, carrots, and celery. Sauté for 5 to 7 minutes until softened.
05 - Stir in garlic and tomato paste; cook for 1 to 2 minutes until fragrant.
06 - Deglaze the pot with red wine, scraping up any browned bits. Simmer for 2 minutes.
07 - Return oxtail to the pot. Add beef stock, bay leaves, thyme, and rosemary. Bring to a gentle simmer.
08 - Cover and transfer the pot to the oven. Braise for 3 hours, turning oxtail once or twice, until meat is very tender.
09 - While oxtail braises, boil potatoes in salted water until tender, about 20 minutes. Drain and return to the pot.
10 - Add butter to potatoes and mash until smooth. Gradually add warm milk, beating until creamy. Season with salt and white pepper to taste.
11 - Remove oxtail from oven. Skim excess fat from the sauce surface. Discard bay leaves and herb stems. Adjust seasoning.
12 - Plate braised oxtail with sauce over a bed of creamy mashed potatoes.