2. Whisk together the eggs and lemon juice until very smooth. Whisk a couple of ladles of the hot broth into the egg then vigorously whisk the eggs back into the saucepan of soup. Switch to a wooden spoon and stir the soup over low heat until it thickens enough to lightly coat the spoon, which should only take a minute. Do not let it come close to a simmer or it might curdle.
3. Remove it from the heat and stir in the chervil. The soup will thicken a little more as it sits for the next minute. Taste for salt. If you used commercially prepared salted broth, it probably won't need more, but if you made the soup with unsalted broth, you'll need to add as much as a teaspoon.
4. Serve the soup right away, or keep it warm by putting the saucepan in a large pot of simmering water, stirring occasionally.
Andrea Reusing is the creator of the restaurant Lantern in Chapel Hill, N.C., and author of the book Cooking in the Moment: A Year of Seasonal Recipes. In this installment of The Key 3, she shares with Lynne Rossetto Kasper the techniques behind three of her favorite recipes: Turnip Soup, Overnight Braised Short Ribs and Tomato Salad.