Remove any loose bone fragments from the oxtail pieces and pat the meat dry with paper towels. Season with salt and freshly ground black pepper, then dust lightly with the flour, shaking off any excess.
Heat the olive oil in a large, heavy-bottomed Dutch oven over medium-high heat. Brown the oxtail pieces on all sides, working in batches so the pan doesn't become overcrowded. Allow approximately 8–10 minutes in total for browning. Remove the meat and set aside.
Reduce the heat to medium. Add the chopped onions, carrots and sliced leek to the same pot. Cook for approximately 8–10 minutes, stirring occasionally, until the vegetables begin to soften.
Add the chopped garlic and cook for another minute, stirring to prevent it from burning.
Stir in the chopped tomatoes and cook for approximately 4–5 minutes, until they begin to break down and release their juices.
Pour in the red wine and scrape the bottom of the pot with a wooden spoon to release all the flavorful browned bits. Allow the wine to simmer for approximately 5 minutes.
Return the browned oxtail to the pot, together with any juices that have collected on the plate. Pour in the beef stock and add the bay leaves and thyme.
Bring the liquid to a gentle simmer, then reduce the heat to low. Cover with a lid and cook for approximately 3–4 hours, or until the meat is extremely tender and pulls away easily from the bone.
Check the stew occasionally, turning the meat and making sure there is enough liquid in the pot. Add a little hot water or extra beef stock if necessary. The sauce should simmer very gently, not boil rapidly.
Once the meat is tender, carefully remove the oxtail pieces and place them on a warm plate. Remove the bay leaves and thyme stem from the sauce.
For a smoother sauce, use an immersion blender to partially or completely blend the cooked vegetables into the cooking liquid. Alternatively, leave the sauce rustic, with the vegetables still visible.
If the sauce is too thin, simmer it uncovered over medium heat for approximately 5–10 minutes, until it reaches your preferred consistency. Skim off any excess fat from the surface.
Return the oxtail pieces to the sauce and warm gently for a few minutes. Taste and adjust the seasoning with salt and freshly ground black pepper.
Serve the rabo de toro hot, with plenty of sauce spooned over the meat. Garnish with a little fresh parsley and thyme, if desired, and serve with mashed potatoes, Spanish fried potatoes or crusty bread.