In a bowl, mix the lemon juice, brown sugar, Dijon mustard, soy sauce, olive oil, garlic, ginger, salt, and pepper. Place the lamb in a shallow container. Pour the lemon juice mixture over the lamb. Cover, and marinate in the refrigerator 8 hours or overnight.
Preheat an outdoor grill for medium heat. Drain marinade, and bring to a boil in a small saucepan. Reduce heat to low, and simmer, whisking constantly, until slightly thickened.
Lightly oil the grill grate. Over indirect heat, grill the lamb 40 to 50 minutes, turning to cook all sides, to a minimum internal temperature of 145 degrees F (63 degrees C). Cool, slice, and cover with the thickened marinade mixture to serve.
