Place the meat on a large plate. Secure the meat to the bone with the kitchen twine. Season with salt and pepper. Place the flour on a deep plate. Dredge the shanks in flour, shaking off excess. Place a rack in the middle of the oven and preheat to 160 °C / 320 °F.
sear the meat
Place a large cast iron pan (oven-safe skillet) over medium heat. Add the olive oil and the first two shanks. Sear the beef for 4 - 6 minutes on each side or until golden and seared. Repeat with the rest of the meat. Transfer the meat to a different plate.
saute the vegetables
Clean the carrots, onion, and celery and cut them into one-centimeter (half-inch) chunks. Add one tablespoon of olive oil and veggies to a cast-iron skillet. Cook for 8 - 10 minutes, then add the tomato paste. Stir and cook for another 2 minutes. Pour in the wine and add the bay leaves. Bring to a boil, then simmer for 3 - 4 minutes for the alcohol to evaporate.
Pour the beef stock into the sauce. Season with salt and pepper, and add the Osso Buco. Cover with a lid and transfer to a preheated oven. Cook for 1 1/2 - 2 hours or until the meat is soft, juicy and until the meat is falling off the bone. When the meat cooks, remove the twine and bay leaves and transfer them to a plate.
Carefully place the skillet with the remaining sauce to medium heat. Cook for 20 - 30 minutes for the sauce to thicken. Add the meat to the sauce right before serving. Serve with your favorite side dish.