Preheat the oven 180°C.
First, spatchcock the poussin. Put the poussin on a chopping board, breast side down. Using a pair of sharp, kitchen scissors, cut along the right side of the backbone, from the tail to the neck. You can cut along the left side too and discard the bone, or leave the bone there.
Turn the poussin over and push it down with your hand on the wings and legs to flatten the bird.
Mix the leaves of 4 thyme sprigs, lemon zest, minced garlic and the butter. Cut the lemon into thin rings and layer them in a large roasting dish.
Rub the bird with the butter mixture, under the skin, over the skin - all over. Place them on top of the lemon slice.
Put two lemon slices and the one thyme sprig on top of each bird, then season them with salt and pepper.
Pour the wine or stock around the bird and bake in the oven for 35 minutes or so, until golden brown.
Cut the birds in half, then serve with side dishes of your choice and the jus (or make gravy from the jus).