Roasting sweet potatoes in a lightly ashed-over bed of coals turns their flesh fluffy and smoky. Drizzle with sour cream, and garnish liberally for a riot of textures, colors, and tastes.Recipe adapted from Slow Fires. Copyright 2015 by Justin Smillie. Published by Clarkson Potter/Publishers, an imprint of Penguin Random House LLC. $40, 320 pages.