Stir together 1/4 cup pomegranate molasses, garlic, rosemary, salt, black pepper, Aleppo pepper, mint, and cumin in a bowl until combined. Set aside.

Lay lamb, fatty-side up, on a work surface. Cut 2 deep slashes in each fleshy end. Flip lamb and cut 10 to 20 shallow slashes across the surface of the fat.

Spread 3/4 of the pomegranate molasses mixture over the flesh-side of lamb. Fold meat together and place into a large bowl. Spread remaining pomegranate molasses mixture over the fatty-side of lamb. Cover the bowl with plastic wrap and refrigerate for 8 hours to overnight.

Preheat the oven to 350 degrees F (175 degrees C). Place a roasting rack in the bottom of a roasting pan.

Remove lamb from marinade and discard excess marinade. Place lamb, fatty-side up, on a work surface. Tie the center section of lamb with kitchen twine to cinch roast together. Repeat ties four times along the length of roast.

Season lamb with salt and place on the roasting rack in the prepared pan. Pour water below the rack to cover the bottom of the pan.

Roast in the preheated oven until hot and slightly pink in the center, about 1 hour 45 minutes. An instant-read thermometer inserted into the center should read at least 135 degrees F (57 degrees C). Rest lamb for 15 minutes, then transfer to a plate and brush with remaining 1 tablespoon pomegranate molasses.