1 To make the puree cook the celeriac and the potatoes in boiling, salted water for about 20 minutes.
2 Heat the oven to 100°C (80°C in a fan oven), 200°F, gas 1.
3 Season the meat with salt and ground black pepper and fry in 2 tbsp hot oil on both sides. Place on an oven rack (with a tray underneath to collect the fat) and roast gently for 6-10 minutes.
4 Fry the onions in the meat juices with the remaining oil until golden brown. Deglaze with the red wine, add a little water and season with the vinegar, salt, and ground black pepper.
5 Drain the vegetables and leave to cool a little. Add the cream, basil and the butter and mash into a smooth puree. Season with salt and nutmeg. Arrange on plates with the steaks and the onions.