Step 1: Butterfly the pork, that is, make a lengthwise cut through one side almost to but not through the other side, holding the knife blade parallel to the cutting board. Open up the pork loin as you would a book. Using the side of a heavy cleaver, a scalloppini pounder, or a rolling pin, lightly flatten the butterflied pork.
Step 2: Generously season the inside of the pork with salt and pepper. Arrange the onion slices on top of one side. Arrange the garlic, if using, on top. Fold the other side over the pork to return it to its original cylindrical shape. Using butchers string looped over crosswise, tie the roast into a tight cylinder.
Step 3: Set up your grill for indirect grilling and preheat to medium (350 degrees F). Toss the soaked wood chips on the coals or place in the smoker box or a smoker pouch if using a gas grill. Cook the pork until crusty and browned on the outside and the internal temperature reaches 180 degrees F, about 1-1/2 hours.
Step 4: Transfer the Spiessbraten to a cutting board and let rest for 5 minutes. Remove the butchers string and carve the roast crosswise into 1/4-inch thick slices. Admire the smoke ring. Serve with Icicle Radish Salad, if desired.