This kale and spinach soup has a beautiful complexity. It's slightly sweet, with a bright note of lemon and the subtle aromatics of thyme, sage and garlic. Japanese yams are marvelously flavorful; they have a dark purplish skin and are snow-white inside. Ask for them at your farmers' market or grocery, but if they are unavailable, regular sweet potatoes can be substituted. Recipe by Anna Thomas for EatingWell.