We took a lamb recipe from Jerusalem: A Cookbook and adapted it with a cooking technique from Cook's Illustrated to create a perfectly tender, medium-rare lamb roast with shawarma spices.

For the lamb marinade

• 5lb leg of lamb, deboned and butterflied
• 2 tsp black peppercorns
• 5 cloves
• ½ tsp cardamom pods (about 5)
• ¼ tsp fenugreek seeds
• 1 tsp fennel seeds
• 1 tbsp cumin seeds
• 1 star anise
• ½ cinnamon stick

• ½ nutmeg, grated
• ¼ tsp ground ginger
• 1 tbsp sweet paprika
• 1 tbsp sumac
• ¾ tbsp sea salt
• 2" fresh ginger, grated
• 3 garlic cloves, crushed
• ⅔ cup chopped cilantro, stems and leaves
• ¼ cup lemon juice
• ½ cup peanut oil (substitute: mix of olive oil and sesame oil)

For the roasting

• 1 large onion, sliced
• 4 cloves garlic, sliced
• 2 large shallots, sliced
• 1 lemon, sliced
• 1 Tbsp olive oil
• 2 tsp sumac
• 1 tsp salt

Create the Marinade

Put the first eight spices in a dry pan and roast on medium–high heat for a minute or two, or until the spices begin to pop and release their aroma; take care not to burn them.
Add the nutmeg, ginger, and paprika and toss for a few seconds.
Remove from heat and cool.
Transfer the spices to a spice grinder and process them to a uniform powder; transfer to a medium bowl and stir in all the remaining marinade ingredients.

Prepare the Lamb

Fat cap side down, use a small sharp knife to trim any pockets of fat and as much connective tissue as you can from the "inside" of the lamb leg.
Flipping back over, trim the fat cap to and even ⅛" all around (or trim the cap completely off, but reserve for putting on top while roasting?).
Pound the roast to an even 1" all over.
Score the remaining fat with slits that go through the fat but not the meat.

Lightly salt both sides of the lamb and let rest for at least 5 minutes, but preferably an hour.
Place the leg on a large cookie sheet and rub the marinade all over; use your hands to massage the meat well.
Cover with foil and set aside for at least a couple of hours, preferably in the fridge overnight.

Roast

Heat the oven to 250°F.
While the lamb is marinating in its post-massage juices, prepare the roasting pan.
In a bowl, combine the sliced onions, garlic, shallots, and lemon with the salt, sumac, and olive oil; stir to coat.
Line the bottom of the roasting pan with these aromatics and roast for 1 hour, or until the alliums are golden brown.

Put the lamb on top of the aromatics with its fatty side facing up.
Return to the oven and roast for 1 hour, or until a meat thermometer in the thickest part registers 135°F
Remove the lamb from the oven, tent with foil, and allow to rest for 20 minutes before carving.

Cut lamb into three equal pieces and then carve cross-wise (against the grain).