Pour the chicken stock into a large saucepan and bring it to the boil. Add the risoni and cook for 5 -10 minutes.
Beat the eggs until frothy, then add the lemon juice and cold water. Slowly stir a ladleful of the hot chicken stock into the eggs, then add one or two more ladles of soup. Be careful not to scramble the eggs!
Remove pan from the heat and stir in the egg mixture carefully into the remaining soup. Season with salt and pepper and serve immediately, garnished with lemon slices. Do not let the soup boil once the eggs have been added or it will curdle.