Preheat the oven to 180C/350F/Gas 4.
For the steak, brush the meat with the oil and season well with salt and freshly ground black pepper. Heat a chargrill pan until smoking, then griddle the steak for 2-3 minutes on both sides (for rare), or until cooked to your liking. Remove from the pan and allow to rest on a warm plate.
For the celeriac chips, place the celeriac slices on a baking sheet and drizzle over the olive oil. Season with salt and freshly ground black pepper, scatter over the garlic cloves and roast in the oven for 12-15 minutes, or until the celeriac slices are golden-brown and crisp.
For the mushrooms, melt the butter in an ovenproof frying pan and fry the mushrooms, whole, for 2-3 minutes, or until lightly golden. Transfer to the oven to roast for 8-10 minutes, or until golden-brown all over.
For the jus, pour the wine into the steak pan to deglaze and reduce over a high heat for 3-4 minutes, or until the liquid has reduced by half.
Serve the steak with the celeriac chips and mushrooms on the side and spoon the jus around the edges of the plate.