For the celeriac purée, heat a saucepan and add the butter, onions, celery and salt. Fry gently for 2-3 minutes.
Add the milk, chicken bouillon and water, then the celeriac and white pepper. Bring to the boil, then reduce to a simmer and cook for 30 minutes.
Remove from the heat and allow to cool slightly. Press the mixture through a colander into a bowl and beat well until smooth. Transfer to a blender and blend to a very fine purée.
Return the purée to the pan, add a little butter and taste to check the seasoning. Set aside and keep warm.
For the madeira cream sauce, heat a frying pan, add 25g/1oz of the butter, the two sliced shallots and a pinch of salt and cook over a low heat until the shallots are transparent.
Add the button mushrooms and continue cooking until they are slippery in texture, then add the thyme.
Add the madeira and simmer until reduced by half.
Add the chicken stock and reduce by half again, then add the cream and reduce this by half.
Stir in the remaining butter, then pass the mixture through a fine sieve into a clean pan and season with salt and freshly ground black pepper. Set aside and keep warm.
For the roasted shallots, heat the butter in a heavy-based pan. Add the blanched shallots and season with salt and freshly ground black pepper. Fry gently until the shallots are softened and coloured.
Increase the heat slightly, add the sherry vinegar to the pan and stir with a wooden spoon, scraping up the bits from the bottom of the pan. Set aside.
For the steaks, season the steaks on both sides with salt and freshly ground black pepper.
Heat a frying pan until very hot. Add the butter, vegetable oil and steaks and cook for 2-4 minutes on each side, depending on how you like your steaks cooked. Remove the steaks from the pan and leave to rest on a tray for a few minutes.
Pour off the excess fat from the pan the steaks were cooked in. Add a splash of water to the pan and pour in the madeira cream sauce. Bring to the boil and reduce to a sauce consistency, then set aside.
In a separate frying pan, heat a knob of butter and gently fry the finely chopped shallot until softened but not coloured. Add the wild mushrooms and fry gently until softened. Add the wild mushroom mixture to the madeira cream sauce.
In the same pan as the shallots and mushrooms were cooked in, heat a little butter and gently cook the spinach until wilted. Season, to taste, with salt and freshly ground black pepper.
To serve, divide the spinach among four serving plates. Slice the steaks and fan out on top of the spinach.
Using a large spoon, place a teardrop-shaped smear of celeriac purée onto each plate.
Place five roasted shallots onto each plate and spoon the mushroom and madeira cream sauce over the meat.
