Prepare your Egg: Stabilize your fire at 400 degrees, as measured at the dome thermometer. Add whatever smoking wood you prefer and allow the Egg to heat up. Place a raised grid in the Egg.
About the lamb:
You can purchase racks of lamb already frenched at Sam's. They come 2 to a package and typically weigh about 12 ounces each.
To prepare the racks of lamb: Heat the vegetable oil in a saute pan over high heat. Sear the racks on all sides until nicely browned. Remove racks to a platter and allow to cool enough to handle.
To make the macadamia crust: Put dried rosemary into a food processor and pulse several times to grind. (If you don't do this the rosemary pieces will be too large. If you find that your food processor just spins the rosemary around, you'll have to chop it by hand.) Add walnuts, salt, white pepper and garlic powder. Pulse several times until mixture is the consistency of bread crumbs. Pour mixture onto a large dinner plate or a platter.
To make the mustard mixture: In a dish combine the egg, mustard and cloves of minced garlic. Mix well with a fork until egg and mustard are completely incorporated. You may wish to prepare this an hour or so in advance in order to allow the flavors to meld.
To coat the racks of lamb: Once the racks are cool enough to handle, place 1 rack in the mustard mixture and coat all sides. (You may find it helpful to use a spoon to get the mustard mixture on the bone side next to the bones.) Next place the coated rack into the macadamia mixture and coat well. Using your hand press the macadamia mixture firmly onto the coated rack. Repeat steps for each rack. You can make the recipe to this point and place in refrigerator until ready to cook.
To cook: Place the racks of lamb on the raised grid, bone side down and roast for 25 - 30 minutes for medium rare. Carefully move from the Egg, so as not to disturb the crust, and let stand for 5 to 10 minutes before cutting or serving.
To serve: As the crust is relatively fragile, we don't recommend that you try to cut the racks into individual chops. You can cut them in half for smaller portions, however. The nuts make this dish quite rich, and a smaller portion goes a long way!