For the crisp shallots, preheat the oven to 140C/120C Fan/Gas 1. Wrap the shallots in a foil bag with the thyme and cook for 45 minutes.
For the chateaubriand, preheat the oven to 200C/180C Fan/Gas 6. Season the beef generously with salt and freshly ground black pepper, heat a dash of oil in a pan and colour the beef over a medium heat until browned on all sides. Add the whole garlic and thyme.
Transfer the beef, garlic and thyme to a roasting pan and cook in the oven for 15–20 minutes or so. Remove from the oven, pour over the Madeira and add the butter. Once the butter has melted, spoon it over the meat, then cover with foil and leave to rest for 10 minutes in a warm place.
For the sauce, cover the garlic cloves with cold water and bring to the boil. Once boiling, remove the garlic from the pan. Repeat this process twice, each time topping with cold water, boiling and removing again. Set aside.
Heat a little oil in a pan and sauté the lardons all over to colour. Add the mushrooms, onions and the reserved whole blanched garlic cloves
Continue to cook slowly until golden-brown all over. Add the port, bay leaf and thyme and cook until the liquid has reduced to a syrup. Add a splash of vinegar and finally the beef stock. Continue to cook until the volume of liquid has reduced by half again.
Meanwhile, blanch the bone marrow pieces in a large pan of boiling water for five minutes. Drain, remove the meat from the bone, cool and chop. Add the bone marrow pieces to the sauce, taste and season with salt and freshly ground black pepper.
To finish the crisp shallots, remove the shallots from the oven, cool, carefully remove the foil, cut the shallots open and peel the skin back gently. Heat the oil in small pan (CAUTION: hot oil can be dangerous. Do not leave unattended.) Deep fry the shallots for a few minutes to allow them to crisp up. Carefully remove using a slotted spoon and set aside.
For the tarragon and parsley oil, prepare a bowl of ice cold water. Blanch the picked tarragon and parsley in a pan of boiling water for 30 seconds. Remove using a slotted spoon and place in the bowl of cold water. Once cool, immediately drain and blend the blanched herbs with the oil in a small food processor or hand-blender until super green and place in a squeezy bottle.
To serve, slice the chateaubriand and roasted garlic in half. Spoon the sauce onto two plates and serve the beef with the crisp shallots and half a head of roasted garlic, all drizzled with a little of the tarragon dressing.