How to Make It
Using the tip of a paring knife, make 1-inch deep incisions all over the lamb, spacing them 1 inch apart. Insert the slivered ginger in half of the incisions and the sliced garlic in the other half. Spread the lamb in a shallow glass or ceramic baking dish.
In a blender, combine the chopped ginger and garlic with the scallion whites and lemon zest and chop. Add the ginger beer, pineapple juice, soy sauce, sesame oil, lemon juice and pepper and puree. Pour the marinade over the lamb and refrigerate for at least 4 hours or overnight, turning twice.
Light a grill. Transfer the lamb to a platter. Strain the marinade into a medium saucepan and boil over high heat until reduced to 1/2 cup, about 10 minutes. Remove the glaze from the heat.
Lightly brush the grate with vegetable oil. Grill the lamb over a medium-hot fire for 8 to 10 minutes per side for medium rare and 10 to 12 minutes per side for medium. During the last 3 minutes of cooking, baste both sides of the lamb with the glaze.
Transfer the lamb to a cutting board; let stand for 3 minutes. Slice thinly on the diagonal and arrange on a platter. Drizzle the lamb with the remaining glaze, sprinkle with the scallion greens and serve.