Bring the chicken stock, lemongrass, lemon zest and juice, herb stalks (use leaves for the threads), peppercorns and 1 teaspoon salt to the boil. Add the chicken breast and let it steep for 20 minutes.
For the threads, mix together the flour, egg, a pinch of salt and milk. Add butter and herbs. Mix with the cutting stick. Bake four thin pancakes in hot clarified butter, roll up and cut into thin slices.
Pass the soup through a fine sieve and season to taste. Cut the chicken breast into pieces.
Place spring onions, strings and chicken on plates, pour hot soup over them and serve.