Heat 1/2 cup of the oil in a heavy-bottomed soup pot or cast-iron skillet. Place the flour in a shallow bowl. Sprinkle the oxtails with salt and pepper and toss them in the flour. Shake off the excess flour and, working in batches, brown on all sides. Remove to a platter as done.
Add a little more oil to the pot if needed and add half the onions and celery. Let cook until beginning to soften, 3 to 4 minutes. Add the garlic and cook 2 minutes more. Add the tomato paste and let simmer for 2 minutes. Add 2 cups of the beef stock, scrape the bottom, stir, and return the oxtails to the pot. Bring to a simmer and add the wine. Simmer on low for 15 minutes.
Add the bay leaves, thyme, 1 teaspoon salt, 1/2 cup of the carrots and the remaining 2 cups beef stock, 1/2 cup celery and 1/2 cup onions and stir. Simmer for 30 minutes. Add the remaining 1/2 cup carrots and simmer for 2 hours. Add the tomatoes; simmer until the broth begins to thicken and the meat falls easily away from the bone, 30 to 45 minutes more. Remove the thyme and bay leaves and adjust seasoning to taste. Put two or so oxtails in each bowl and ladle the soup over them. Serve with the bread.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.