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Green Soup with Yams & Sage
“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.”
3 tablespoons extra-virgin olive oil, divided, plus more for garnish
2 large onions, chopped
1 teaspoon salt, divided
2 tablespoons plus 4 cups water, divided
4 cloves garlic, sliced
1 teaspoon chopped fresh thyme leaves or ½ teaspoon dried
1 large bunch Tuscan, lacinato or Russian kale
2 medium or 1 very large Japanese yam or regular sweet potato (about 1¼ pounds)
1 tablespoon agave nectar, or more to taste (optional)
16 fried sage leaves for garnish (see Tip)
1Heat 2 tablespoons oil in a large skillet over high heat. Add onions and ¼ teaspoon salt; cook, stirring frequently, until the onions are beginning to brown, about 5 minutes. Reduce the heat to low, stir in 2 tablespoons water, garlic and thyme and cover. Cook, stirring frequently until the pan cools down, and then occasionally, always covering the pan again, until the onions are greatly reduced and have a deep caramel color, 25 to 35 minutes.
2Meanwhile, remove tough stems and ribs from kale and coarsely chop the greens. Peel yam (or sweet potato) and dice into 1-inch pieces. Coarsely chop spinach; set aside.
3Combine the remaining 4 cups water and ¾ teaspoon salt in a soup pot or Dutch oven; add the kale, yam (or sweet potato) and sage. Bring to a boil. Reduce heat to maintain a simmer, cover and cook for 15 minutes.
4Stir in the spinach, return to a simmer, cover and cook, stirring once halfway through, for 10 minutes more. When the onions are caramelized, stir a little of the simmering liquid into them; add them to the soup. Add broth; return to a simmer, cover and cook for 5 minutes more.
5Puree the soup in the pot with an immersion blender until perfectly smooth or in a regular blender in batches (return it to the pot). Stir in cayenne, a few grinds of pepper and 1 tablespoon lemon juice. If the soup is sweet to your taste, add more lemon juice; if it's too tart, add agave nectar, if desired. Just before serving, whisk the remaining 1 tablespoon oil into the hot soup. Garnish each bowl of soup with a drizzle of oil and 2 fried sage leaves.
Tip: To make fried sage leaves: Set a small strainer over a heatproof bowl. Heat about ½ inch olive or canola oil in a small saucepan over medium-high heat until shimmering but not smoking. Add sage leaves; fry just until crisp, 1 to 3 minutes. Drain in the strainer then spread out on a paper towel until ready to use.
To make ahead: Cover and refrigerate for up to 3 days.