First peel the celeriac down to the white, smooth flesh. Cut into rough 1-inch (2 centimeter) dice. Half fill a large saucepan with salted water and bring to the boil. Take another pan, which will snugly fit your 4 steaks later, and add your chicken stock, wine, garlic, bunch of thyme, peppercorns and a pinch of salt. Bring this to the boil, then lower the heat to a gentle simmer. By this time your pan of boiling water should be ready, so add your celeriac, cover with a lid and boil fast for around 15 to 20 minutes until it's tender.

Place your 4 steaks side by side into the simmering wine and stock, making sure they are covered well by the liquid. Add a little water if needed, but if the pan is nice and snug the steaks should be well covered. As soon as they are in the pan, it generally takes about 6 minutes to cook rare, 8 minutes for medium and 10 for medium to well done. Depending on the thickness of the steaks and how cold they were when you put them in, there's always a bit of leeway on the timing, so the best thing to do is give them a little pinch to check how soft they are in the middle. Cover the pan with a cartouche of greaseproof paper.

When the steaks are cooked to your liking, remove them to a warm plate, cover and leave them to rest for a couple of minutes while you drain your cooked celeriac in a colander. Place it back in the pan, adding half the butter. Mash to a smooth puree and season well to taste.

To make a light sauce, all you need is 2 wineglasses full of the poaching liquor - you can freeze the rest. Bring this to a fast boil, then remove the thyme and peppercorns and mash up the garlic, which will be soft and sweet. Allow to boil for a couple of minutes. Remove from the heat, season to taste and add the rest of the butter. Shake the pan lightly so the melted butter dissolves into the wine - this will make a shiny lightly thickened sauce. Do not re-boil as it will split. Serve each steak with a bit of mash and a little sauce.