Combine the lavender, lemon zest and juice, fresh thyme leaves, olive oil and honey in a bowl and whisk briefly to form your marinade.
Place the poussin in a large bowl, or if you would prefer, a sealable sandwich bag. Use a tablespoon to spoon over the marinade, then use your hands to rub the marinade all over the poussin. Make sure you get a nice even coverage. Lift the skin on the poussin’s back and place a knob of butter underneath. Repeat this in a couple of other places under the skin.
Place the poussin in the fridge to marinate for at least 2 hrs. (ideally up to 24 hrs., the longer the better).
When you are ready to cook your poussin make sure you bring it out of the fridge for 30 min. first to get it back up to room temperature. During this time you can preheat your oven to 220 °C/430 °F.
When the oven is up to temperature, transfer your poussin to a roasting tray, season with salt and pepper, and place on the middle shelf of the oven.
Roast for approx. 40 min. at 220 °C/430 °F. Before removing entirely, check if the poussin is cooked by inserting a small sharp knife or a metal skewer between the leg and the breast. If the juices run pink then give the poussin an additional 5 – 10 min. in the oven and check again, only serve when the juices run clear.
Let the poussin rest for 7 – 10 min. in the roasting pan with a piece of tin foil covering it to keep the heat in. Transfer to a chopping board, carve and serve.