Mix the 2 cups red wine, red wine vinegar, and the Dijon mustard in a bowl. Add the oxtail and mix until oxtail is fully coated. Refrigerate for at least 12 hours.
Preheat the oven to 350°F.
Remove the oxtail from marinade and discard the liquid.
In a Dutch oven or heavy-bottomed pot, heat the olive oil over medium-high heat. Sear the oxtail until browned on all sides. Add the diced vegetables and let cook for five minutes. Add the tomato paste and one cup of red wine, scrapping the bottom of the pot to remove any stuck vegetable bits.
Add beef broth and bring to a boil. Remove the pot and place in the oven for 3 hours, or until meat is falling off the bone.