To make the lamb sauce, preheat the oven to 220C/200C Fan/Gas 7. Spread the lamb bones out on a baking tray, or multiple trays if necessary. Roast in the oven for about 50 minutes until the bones are deep brown and caramelised.
Put the olive oil, onions, carrots, celery and garlic in a saucepan and cook over a medium heat until caramelised and all liquid has evaporated. Add the white wine to the pan and reduce until it reaches a syrupy consistency. Add the chicken stock and 6kg of roasted lamb bones. Bring to a boil, skim any foam and impurities that come to the top, then add the tomatoes and mushrooms and simmer for 2 hours, skimming regularly. Pass the stock through a fine chinois into a clean pan. Add the veal reduction and reduce on a rolling boil, until a sauce consistency is achieved. Add the remaining 2 kilograms of lamb bones and the thyme and soy sauce, and let sit for 30 minutes. Pass through a fine mesh sieve and keep warm until ready to plate, or refrigerate if you’re making it ahead of time.
To make the basil purée, cook the garlic in the olive oil in a saucepan until soft. Add the courgette slices and cook for 5 minutes, then add the basil and cook for another 5 minutes. Transfer to a blender and blend until smooth. Add salt to taste and chill immediately. Reserve until ready to plate.
To make the chicken and basil mousseline, place the chicken into a food processor and blend until broken down and smooth. Try not to blend for too long or the chicken will become warm.
Add the egg white and salt and blend again to mix. Pass the chicken through a very fine drum sieve into a bowl, a small amount at a time, using a scraper to push it through the mesh. Work in small batches, as trying to sieve too much at once will overwork and warm up the chicken. Once it is all sieved, weigh it. You will need three-quarters of this weight in cream – it should be approximately the amount listed in the ingredients. Cover the chicken and chill for about 30 minutes, until cold.
Add 1–2 tablespoons of chilled cream at a time to the chicken, beating it in quickly after each addition using a spatula. It must be added gradually or the mousseline will be too thin to hold its shape. Once all the cream has been added, add the basil, raw sweetbreads and the haggis, beating to incorporate. Poach a little of the mousseline in simmering water to test for seasoning, then taste and add salt and white pepper, as necessary. Spoon the mixture into a piping bag and chill until ready to use.
For the lamb, remove the eye of the loin (also called the lamb pencil) from the rest of the lamb loin. Trim away any sinew and wrap it in nori sheets. Trim the rest of the loin down, leaving enough meat to wrap all the way around the outside of the lamb pencil.
Pipe the chicken mousseline inside part of what will become the lamb roll, where the eye of the loin used to sit. Make sure you keep enough of the mousseline to stuff the 8 courgette flowers later. Now return the lamb pencil to the inside of the loin and wrap the meat around it, with the mousseline in between. Tie the lamb with butcher’s string so it’s nice and tight. Season the outside with salt and pepper.
Preheat the oven to 220C/200C Fan/Gas 7. Place the lamb on a baking rack in the oven. Ensure you have a tray underneath to catch the juices. After 8 minutes, or once the fat is crispy, open the oven door and reduce the temperature to 60C. Keep the oven door open for about 5 minutes to dramatically reduce the heat. The meat will still keep cooking as the outside is still really hot. Once the oven door is closed, cook for a further 25-30 minutes. Check the temperature with a probe. Cook until the core temperature reaches 52C. Once the desired temperature has been reached, take the meat out of the oven and rest on a baking tray for 15 minutes.
Pipe the rest of the chicken mousseline into the courgette flowers, then steam them for 4 minutes to cook.
To prepare the garnish, heat a small frying pan and add the olive oil. Once hot, add the courgettes and cook on both sides until coloured.
To serve, place dots of basil purée onto a serving plate, along with the fried courgettes, carved saddle of lamb, courgette flowers and lamb sauce. Finish with the picked chervil and monk's beard.