Warm the oil in a deep, lidded pan, add the chicken pieces, skin side down and lightly seasoned, then cook until the skin is a rich, deep gold. Turn them and colour the underside similarly, then remove from the pan and set aside.
Set the oven at 200C. Trim the leeks, discarding the roots and very tips of the leaves. Cut into short pieces about the length of a wine cork, then wash them thoroughly, removing every grain of grit trapped between their layers.
Pat the leeks dry, then fry them in the chicken pan over a moderate heat for about 5 minutes, covered with a lid, so they soften without browning. Pour in the stock and bring to the boil, stirring as you go, then add the half lemon, peppercorns and a teaspoon of salt. As the stock boils rain in the orzo and continue boiling for 3 minutes. Return the chicken to the pan, cover again and bake for 35 minutes.
Remove the lid, add the peas, then return to the oven for 5 minutes. Check the seasoning, then add the tarragon and parsley and serve in bowls, with spoons.