Still in cryopack? What I would personally do is one of 2 things.
Lazy way - remove all the paper from the outside of the cryopack, toss in the sous vide bath at 135 overnight or until we decide we want to start dinner, but at least 6 hours, overnight better. Cook up some form of veggitude - mashed potatoes, a good tossed salad, and a nice boule of sourdough bread is good. Take the cooking juices out of the bag and turn into gravy with the addition of some good defatted chicken stock if it needs it [we find that cooking meat that is well marbled makes for some good juices int he cryopack, especially if we do a flat of pork ribs] if we do a roux, a good flour and butter golden roux works the best. Cornstarch or arrowroot if you are really lazy.
Long way - Get out the good roasting pan [likethis, my mom got it back in the 50s, but instead of a a rack it has a sort of grid with half inch spacing] get a bag each of baby potatoes, boiling onions, baby carrots, a bunch of celery, a small bunch of thyme, parsley, rosemary, a couple lemons and a couple bulbs of garlic. Prep the veggies, take one bulb of garlic, one sprig of rosemary and reserve. Make a bed of the veggies and herbs, and take the roast out of the bag, dump the juices [if any] into the bottom of the roasting pan, poke holes and stick a sliver of garlic or blade of rosemary into each hole. Juice the lemons onto the roast, sprinkle with some salt and pepper and add a couple cups of a mix of 1:1:1 olive oil, lemon juice or dry white wine and chicken broth to the bottom of the roasting pan. Pop in at 425 F and roast for about 45 minutes or so, basting every 10 minutes or so. Uncover, keep roasting another 15 minutes, then take the roast out to rest while the veggies get to cook another 15 minutes recovered. Take and bowl up the veggies, slice the lamb and serve with gravy made from the pan juices.
I might consider chopping it into chunks and making a good stew/curry/soup … we tend to have 3 or 4 cryos of lamb in the freeze at any given point [fully bone in, semiboneless or boneless, I think there is still a rack of chops right now] but we are untypical for most Americans - we also tend to have a couple geese, a couple ducks, I just added a couple pair of venison medallions and found a pound each of ground ostrich, bison and kangaroo. We tend to be moderately adventurous when eating. [oh, and a beef heart, a couple pork hearts and a tub of about a pound of duck hearts from the Chinese grocery in Hartford CT]