Put the macadamia nuts on a cookie sheet or some other shallow pan, and put it in the oven to toast the nuts. Check it every few minutes to make sure you're not over toasting them (all totalled it should take around 6 or 7 minutes). Remove from the oven, and set aside to cool. And drop the oven temperature to 375°
Set a pot of water on the stove to boil. Now start peeling the turnips, then cut them each in half lengthwise. With either a mandolin or a nice sharp knife, thinly slice the turnip. Once your water is boiling, put the turnip slices in and allow to cook for 4 - 5 minutes, so as to allow them to blanch. When that time is up, drain the pot, and put your turnip slices into a bowl with ice water (so as to stop them cooking).
Take your butter, and cut it into smaller portions (so it will melt faster). Put it in a small sauté pan, over low heat. As it starts to melt, mix in the ground ginger. When the butter is entirely melted, turn off the flame.
With a knife, chop up the toasted macadamia nuts. It doesn't have to be perfectly finely chopped, but you want to get them good and broken up, as they're going to be sprinkled in between the layers of turnip.
Now that you've got all of that done, it's time to make the Napoleons. If you have a pastry brush, break it out now - otherwise, you can use a spoon. Coat the bottom of a cake tin (or pie tin) with your ginger butter. Lay in 8 turnip slices, and brush each with ginger butter, then add another layer of turnip, and again with the butter. Sprinkle some of the nuts on each, then another layer of turnip, butter, nuts, turnip, butter, nuts, turnip, butter, nuts, turnip, butter, turnip, butter, and finally a few nuts on top, and drizzle butter over the top of each.
Pop the pan in the oven, and let cook for 40 - 50 minutes. As the turnips are already cooked, you're just trying to soften them more, get the ginger taste to spread out, and crisp the edges really.
To serve, take a spatula and place two Napoleons back to back on a plate, curved sides in.