1large yellow potato, peeled and diced, (6 ounces)
lemon juice, to taste
3/4cupyogurt (whole or low-fat), or crème fraîche
zest of 1 small lemon (meyer or regular)
Fill a pot with 2 quarts of water and bring to a boil. Snap the woody ends of the stems off of the asparagus spears and discard; they will naturally break at the right spot. Keep the heads in tact and slice the stems. Blanche the asparagus in the simmering water for 1-2 minutes, until bright green and crisp-tender. Strain through a colander and into a large bowl, reserving the asparagus water. Rinse the asparagus under cool water to stop the cooking, and set aside.
Trim the root ends off of the green garlic and leeks. Slice the green garlic (leafy green parts includecrosswise into thin rounds, and place the rounds in a large bowl. Slice the leeks in half lengthwise, then slice the white and light green parts crosswise into 1/4" thick half-moons. Place the sliced leeks in the bowl with the green garlic, fill with cool water, and separate the rings, swishing occasionally as they soak to release any sandy dirt, which will sink to the bottom. Scoop the leeks and garlic out of the water (you can repeat the soaking/scooping process if the leeks are super dirty), drain them in a colander and give them a final rinse.
Melt the butter and olive oil together in a soup pot or dutch oven over medium heat. Add the leeks and green garlic and saute until bright green and tender, 5-10 minutes. Add the salt, potato, and enough of the reserved asparagus cooking water to cover the vegetables, 4-6 cups. Bring to a boil, then cover the pot and simmer until the potato is very tender, 10 minutes.
Remove the lid and let the soup cool to warm, 30-60 minutes. (This is to preserve the color of the asparagus, but you can skip the cooling step if you prefer.)
Add the asparagus, reserving several pretty heads for garnish, and puree the soup with an immersion blender, or in batches in a regular blender, thinning with more asparagus water as needed. Stir in the juice of half a lemon, taste for salt, adding more salt or lemon as you see fit.
Meanwhile, stir together the yogurt and lemon zest.
Re-warm the soup, and serve in bowls with a dollop of lemony yogurt and a couple of asparagus tips. The soup will keep in the fridge for up to three days.
The cooking method described below preserves the green hue of the soup; if you prefer, you can skip the blanching and cooling steps and simply add the asparagus with the water and potato.I think fat asparagus stalks tend to be more meaty and creamy, and make the best soup, but any tasty, in-season asparagus will do.Feel free to garnish the soup with chives, chervil, tarragon or cilantro; see the post above for more ideas.Nutritional values are based on one of six servings.