First cut the backbone out of each poussin. Place the birds, breast-side down, on a chopping board and, using a sharp pair of kitchen scissors or shears, cut along either side of the bone to remove completely. Turn over and flatten the poussins by pressing down with your hand over the breast bone.
Combine all the marinade ingredients and place in a large, resealable freezer bag with the poussins. Seal the bag and leave to marinate in the fridge for about 24 hours.
When you’re ready to cook, preheat the oven to 200C/180C Fan/Gas 6.
Take the poussins out of the bag, reserving the marinade to make the sauce. Arrange the poussins, breast-side up, in a large roasting tin and drizzle over the honey.
Place in the oven to roast for about 20 minutes. Measure the cornflour into a jug, add the marinade and mix until smooth. Pour the marinade over the poussins and return to the oven for a further 15 minutes.
Remove the tin from the oven and cut each poussin in half along the breast bone. Strain the juices left in the tin to make a sauce, seasoning with salt and pepper to taste. Serve half a poussin per person, with some of the sauce spooned over and garnished with sage leaves.