Place the cleaned chicken in a deep saucepan. Cover with cold water and bring to a simmer. Skim any scum off the top before adding the thinly sliced onion, the garlic, herbs, spices, lemon zest and stock cube. Bring back to a very gentle simmer and cook for 50 minutes. Turn off the heat, cover and leave the chicken in the stock for 20 minutes.
Melt half the butter in a heavy saucepan an