Onions and sage, a classic pair, give this beautiful soup its subtle character. The sweet potatoes and stewed garlic add a layer of sweetness under the earthy taste of Russian kale, one of my favorite greens — but it is the perfume of the sage that brings it all together.

Steps

1. Peel and dice the sweet potatoes, combine them in a large soup pot with 3½ cups (800 milliliters) water, a teaspoon of salt, and the sage, and bring to a boil. Lower the heat and simmer, covered, about 10 minutes.
2. Wash the kale and chard, trim away the tough stems, and chop the greens coarsely. Add the greens to the soup, along with the garlic cloves and the vegetable broth. Continue simmering gently, covered, for another 20 minutes.
3. Meanwhile, chop the onions and sauté them gently with a pinch of sea salt in the olive oil over medium-low heat, stirring frequently, until they are soft and golden brown, 30 to 40 minutes. When the onions are ready, add them to the soup and let it cool slightly.
4. Purée the soup in a blender, in batches, and return it to a clean pot, or use an immersion blender. Add a little more water or broth if the soup is too thick to pour easily from a ladle. Taste and correct the seasoning with a bit more salt, if needed, and freshly ground black pepper to taste. If you like, add a small amount of fresh lemon juice, but taste as you go — you don’t want to overwhelm the lovely flavor of the onions and sage.
5. When you serve the soup, drizzle a thread of fruity green olive oil on top of each steaming bowl — just a teaspoon or so. This last step is essential, as the taste of the fresh, unheated olive oil is entirely different from that of cooked oil, and it wakes up all the flavors and makes them sing.