Mash together the butter, lemon rind, oregano, garlic and seasoning. Lift the skin from the poussins and slide the flavoured butter between the flesh and skin, or, if you prefer, smear the butter over the skin.
Put the poussins side by side in a roasting tin and cook in a preheated oven, 220°C (425°F), Gas Mark 7, for about 55 minutes, basting occasionally, until golden and crispy and the juices run clear. Remove from the oven and leave to rest for 5 minutes.
Transfer the poussins to a chopping board and use a long, sharp knife to cut each one carefully in half lengthways. Serve immediately with the salad leaves.