Preheat the oven to 180°C.
Brush the steak with oil and season with salt and pepper.
Griddle the steak in a smoking hot pan for 2–3 minutes per side and let it rest.
Place celeriac chips on a baking sheet, drizzle with oil, season, and scatter garlic on top.
Roast celeriac for 12–15 minutes until golden-brown and crisp.
Melt butter in an ovenproof pan and fry mushrooms for 2–3 minutes.
Transfer mushrooms to the oven to roast for 8–10 minutes.
Deglaze the steak pan with red wine and reduce by half over high heat.
Serve steak with celeriac chips and mushrooms, spooning the jus around the plate.