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The day before you want to have the poussin, remove it from its packaging, cutting away the string tied round it, and place in a large freezer bag or baking dish. Combine the buttermilk, lemon zest, half the garlic (save the rest for the polenta chips), the honey, mustard, the leaves from 1 thyme sprig, a small pinch of salt and a good grinding of black pepper to make a marinade. Pour into the bag, then seal it and squish everything around so that the poussin is coated in the marinade – if using a baking dish, coat the bird in the marinade, then cover with cling film. Chill overnight.
To make the polenta chips, line a 2lb loaf tin with cling film. Cook the polenta following pack instructions, then stir in the Parmesan, remaining garlic, the rosemary and season well. Tip into the tin, spread out and smooth the surface. Leave to cool, then cover and chill for at least 30 mins (can be left in the fridge overnight).
Heat oven to 200C/180C fan/gas 6. Remove the poussin from the marinade and sit in a roasting tin. Stuff with the lemon quarters and remaining thyme sprigs. Drizzle with oil, season and tie the ends of the legs together with string. Roast for 35-40 mins, basting every so often. Meanwhile, turn the chilled polenta out onto a chopping board and cut into fat chips – you should have 10. Heat half the oil in a large frying pan, add some of the polenta chips and cook, turning every so often, until really crisp, then transfer to a plate lined with kitchen paper. Repeat with the remaining chips, adding more oil as needed. Transfer to a baking tray. Remove the poussin from the oven and leave to rest in the tin for 20 mins. Keep the polenta chips warm in the oven while the poussin is resting.
Sieve the juices from the poussin roasting tin into a small saucepan. Remove the top layer of fat, then bring to a simmer. Add the Sherry vinegar, heat through, taste for seasoning, then transfer to a serving dish. Sprinkle the polenta chips with salt. Serve the poussin on a plate with some lamb’s lettuce and the chips, with the poussin juices and some mayonnaise alongside for dipping the chips.