First, make the 'butter'. Put all ingredients into a medium bowl and mix together well. Spoon onto a small sheet of parchment paper, roll into a log and chill until firm. This can be frozen ahead of time and thawed when needed.
For the purée, put the celeriac into a pan, half cover with water and then lay the cauliflower florets over the top. Place a lid on the pan and cook for 10-12 minutes or until tender. Drain the water from the pan and let it sit for a couple of minutes, with the lid on, to evaporate any excess liquid. Alternatively cook in a steamer. Now transfer to a food processor, add the horseradish, duck fat and sea salt, then blend until smooth. Mind your eyes when you open up the lid of your processor, if they are sensitive to newly chopped onions you may feel the same here too. Set the mash aside whilst you cook the steak.
Heat a heavy frying pan until hot. Put the avocado oil onto a large plate, place the steaks over the top and turn to thinly coat on both sides. Sprinkle a little salt over the flesh and place into the hot frying pan. Turn the heat down to medium and leave for 3 minutes without moving or the meat will stick. Turn the steaks over and continue to cook for a further 2-3 minutes. This should give you a medium rare cooked piece, but bear in mind yours may be a different size and thickness to mine and may need more or less cooking time. Remove the steaks from the pan and allow to rest for 4-5 minutes on a large warmed plate.
Put a couple of large spoonfuls of the purée onto each plate, top with the steak and a thick round of the thyme 'butter'.
Recipe by Autoimmune Wellness at https://autoimmunewellness.com/rib-eye-steak-celeriac-horseradish-puree-thyme-butter/