Step 1: Make miso rub
Grate (on a microplane or ginger grater) or food-process the onion and garlic until you have at least half a cup of extremely fine puree. Make sure not to lose any of the onion juice. Mix in roughly an equal amount of miso paste, some freshly ground black pepper, and a dollop of olive oil. The result should be a nice brown color.
Step 2: Rub down turkey
Starting at the back of the bird (larger cavity) run your hands under the skin of the breast, and spread your fingers to detatch the skin from the meat. Wiggle your hand around to the leg/thigh area, detatching as you go. Spin the pan and do the same thing from the front of the bird. If this doesn't look positively indecent, you're not doing it right.
Scoop up some miso rub, and shove it in under the skin. It's fine to leave big miso lumps; you can massage the skin from the outside to spread it properly. Work about 75% of the rub in under the skin, trying to reach as much of the bird surface as possible. Flip the bird over, and rub a bit more into the surface of the skin on its back, then return to original belly-up position. Rub remaining miso onto the outside of the breast and leg skin.