Preparation:

Preheat the Oven: Set your oven to a low temperature, around 325°F (165°C). The lower heat helps in gently warming the lamb without drying it out.

Room Temperature: Remove the roast lamb from the packaging and let it rest at room temperature for about 30 minutes before reheating. This helps the lamb to reheat more evenly.

Add Moisture: Place the lamb leg in a roasting pan. You can add a small amount of broth or water to the bottom of the pan. The liquid will create steam during the reheating process, helping to keep the lamb moist.

Cover with Foil: Cover the lamb leg tightly with aluminum foil. This traps the steam, keeps the moisture in, and prevents the outer layer from drying out.

Reheat Slowly: Place the lamb in the preheated oven. The time needed for reheating will depend on the size of the lamb leg. It might take anywhere from 30 minutes to an hour. The goal is to heat it through without overcooking.

Check Temperature (Optional): If you're using a meat thermometer, the internal temperature of the lamb should reach about 145°F (63°C) for medium-rare. Adjust the time if you prefer your lamb at a different level of doneness.

Rest Before Serving: Once reheated, let the lamb leg rest for about 10 minutes before carving. This allows the juices to redistribute throughout the meat, ensuring it's moist and flavorful.

Tips:

Avoid High Heat: High temperatures can cause the lamb to dry out and become tough.
Keep It Covered: Ensuring the lamb is covered throughout the reheating process helps retain moisture.
Use Liquid Wisely: Adding a bit of broth or water helps in maintaining moisture but avoid too much liquid as it can change the texture of the meat.

By following these steps, you should be able to reheat your roasted bone-in lamb leg while keeping it moist and delicious, closely resembling its freshly cooked state.