Before anything, take the Shank out of the fridge and allow to temper for 1 hour (or, budget an extra 10 mins in the water until it's heated through). Bring a pot of water up to a boil, then reduce to a simmer. Insert your Lamb Shank still in the bag. Allow it to heat through - roughly 10-12 mins. To do a quick check, remove the bag from the very hot water and pinch the meat. It should be squishy and tender. Technical terms.
Next step, cut open the bag, pouring just the sauce into the pan. Then gently remove the shank and place it in the pan with the sauce. Turn onto medium heat and with a spoon, continue to coat your lamb with the sauce as it reduces. Once the sauce can coat the back of a spoon, it is good to go. If the sauce gets too thick, add a touch of water.