Step 1

In a large pot, melt the butter over medium heat. Add the leek, celery, and garlic; season with salt and pepper. Cook, stirring occasionally, until the vegetables are tender, about 8 minutes. Add the stock; season. Bring to a boil over high. Add the orzo. Reduce heat to medium. Cook until the orzo is just al dente, stirring often, about 6 minutes. Remove from heat. Stir in the chicken.

Step 2

In a medium bowl, whisk the eggs and lemon juice. Slowly pour 1 cup of the soup into the egg mixture, whisking constantly. Gradually stir the egg mixture into the soup, then stir in the scallions. Return the soup to low heat. Simmer, stirring occasionally, until the liquid thickens, about 5 minutes; season the soup.

Step 3

Divide the soup among bowls. Top with the celery leaves, lemon zest, and dill. Serve immediately.