Bring the chicken stock to a boil in a large saucepan; add salt to taste. Meanwhile, in a medium bowl, beat the eggs with a whisk or electric mixer until thick and frothy. Slow add the lemon juice, beating constantly to prevent eggs from curdling. Stir the egg-lemon mixture into the hot chicken broth; let cool and refrigerate.
Grate or julienne the zucchini; blanch in boiling salted water for 45 seconds just to set the color and remove raw taste. Quickly drain and run under cold water until cool to the touch; refrigerate.
To serve, mix the zucchini into the soup. Serve chilled; garnish with lemon slices.