Chop the onion, carrot, and celery into ¼-inch dice (0.6 cm). It’s important to cut the veggies small so they’ll soften adequately in the sauce. Mince the garlic clove.
Pat the meat shanks dry with paper towels and sprinkle with salt and pepper. You can tie the meat to the bone with kitchen twine to help it hold together (but I never do this).
Dredge the meat in flour, making sure both sides have a thin coating.
Heat 1 tablespoon olive oil in a skillet or Ninja Foodi (using high sauté). Add the osso buco meat and brown for 3-5 minutes per side. Set the beef shanks aside.
Add the other tablespoon of oil to the skillet or Foodi and add the carrot, celery, and onion. Soften the veggies for 2-3 minutes. You are not looking to brown them.
Add the tomato paste, wine, stock, rosemary, thyme, and garlic to the veggies in the skillet. Allow this mixture to cook an additional 2-3 minutes. Let the sauce cool in a bowl and clean out the Ninja Foodi (if that’s what you’re using to sous vide).
Fill your sous vide machine with water an allow it to come to 175°F (79.4°C). Filling my Foodi with water to the halfway mark (about 18 cups) was perfect for sous vide. The bag was fully submerged without the water level rising above the “max” line.
Place the cooled sauce and the meat in a gallon-size freezer baggie with a zip top. Alternatively, seal the food in a vacuum-sealed bag using a vacuum sealer.To sous vide using a freezer bag, you need to make sure the air is pressed out. Put the food in the bag, and partially seal it while pressing out as much air as possible. Next, partially submerge the bag in the hot water, making sure the food is under the water level. When you partially submerge the baggie, the rest of the air should move out. Then you can finish sealing the bag and completely submerge it in the water. Be careful not to burn your fingers!
If using the Ninja Foodi, put the lid on with the vent open to sous vide. Sous vide beef osso buco for 24 hours at 175°F (79.4°C). (Sous vide veal osso buco may have a shorter cooking time.)
After the cooking time is up, carefully remove the beef shanks from the bag and let them rest 5-10 minutes. (As an extra level of precaution, you can check the meat with a meat thermometer to ensure it has reached 145°F/63°C.) Pour the sauce in a skillet or empty Ninja Foodi and sprinkle xanthan gum over it.
Whisk the xanthan gum in to thicken the sauce. The sauce should thicken within 3-5 minutes on the high saute setting.
To serve, mix together fresh finely chopped parsley and lemon zest (if using). This is our gremolata, a condiment that adds nice color and flavor. Plate the meat, add the sauce, and finish with the gremolata. Enjoy!