Chateaubriand with wild mushrooms

For the beef: preheat the oven to 190C/170C fan/Gas 5. Season the beef with salt. Heat a little butter in a heavy-based ovenproof frying pan and, when foaming, add the beef. Sear until nicely coloured on all sides, using tongs to help you turn the meat. Transfer the chateaubriand to the oven and cook to your requirements – 15 minutes for medium-rare and 5 minutes more for a medium steak. Remove from the oven and leave to rest.

For the red wine sauce: heat the oil in a shallow pan and lightly colour the beef trimmings. Add the shallots and sweat for 3 minutes without colouring. Add the mushrooms and, once they appear slippery, pour in the wine. Simmer until the wine is a quarter of its original volume, then add the chicken stock, demi-glace, herbs and peppercorns. Bring to the boil, reduce to a simmer and cook for 1 hour.

Pass the sauce mixture through a sieve then return to a clean pan and simmer until it has reduced to a coating consistency. If needed, mix the cornflour with 1 tablespoon of water and add this to the sauce, stirring until thickened. Taste and season as necessary with salt, pepper and a pinch of sugar if the sauce seems too acidic. Keep warm.

For the celeriac purée: melt the butter in a saucepan and sweat the onion and celery together with 3g salt. When tender but not coloured, pour in the milk and 250ml of water. Add the stock cube, celeriac and 1 pinch of ground pepper then bring to the boil. Reduce the heat to a simmer and cook for 30 minutes. Set aside cool.

Drain the celeriac mixture in a colander then place in a food processor and blend until fine. Transfer the mixture to a blender and blend again to a very fine purée. Put the purée in a clean saucepan and keep warm.

For the vegetables: peel the shallots, leaving them whole. Blanch them in a pan of boiling water for 1 minute, then drain and refresh in iced water. Drain thoroughly, then cut the shallots in half.

Heat a large knob of butter in a heavy-based pan. Add the shallots and season with sea salt and freshly ground black pepper. Fry gently until the shallots are softened and coloured. Increase the heat slightly and add a little sherry vinegar to deglaze the pan, stirring with a wooden spoon to scrape up the bits from the bottom of the pan.

Heat another pan with a knob of butter and, when foaming, add the mushrooms, garlic and chervil and toss well. Fry until just starting to soften, then season generously.

To serve, spoon the celeriac purée onto each plate. Carve the chateaubriand on the diagonal, cutting against the grain and add to the plates. Scatter the beef with the mushrooms, put the shallots on the side and finish with a drizzle of the sauce.