Bring the stock to the boil, add the pasta and simmer for 8–10 minutes until just tender. Meanwhile heat the butter in a separate smaller pan, stir in the flour then gradually mix in 2 ladlefuls of the stock from the large pan. Bring to the boil, stirring. Take off the heat.
Mix the egg yolks in a medium-sized bowl with the lemon rind and some salt and pepper. Gradually mix in the lemon juice until smooth. Slowly mix in the hot sauce from the small pan, stirring continuously.
Stir a couple more hot ladlefuls of stock into the lemon mixture once the pasta is cooked, then pour this into the large pasta pan. (Don't be tempted to add the eggs and lemon straight into the pasta pan or it may curdle.) Mix well, then ladle into shallow soup bowls and top with shredded chicken taken off the carcass, some extra lemon rind and some torn oregano leaves. Serve with lemon wedges.