Melt 50g of the butter in a heavy based saucepan and gently fry off the celeriac. Once it has taken on a little colour add the stock and simmer away gently.
Once the stock has reduced and the celeriac is fully cooked mash the celeriac and stock together to produce a coarse puree. Season to taste.
Season the pincanha with olive oil, freshly ground black pepper and sea salt. Place fat side down in a large heavy based frying pan and heat to start to gently render the fat. This process will take 10-15 minutes on a low to medium heat.
Once the fat is golden and nicely rendered drain off some of the rendered fat and flip the pincanha over to sear the meat, turning the heat up to get some good colour.
Add the second half of the butter and baste the pincanha gently cooking the whole piece of meat. If you are in a hurry you can now place the whole thing in a hot oven to cook for a further 5-10 minutes. However with a little more patience it is possible to cook the whole thing in the pan occasionally turning and rotating the meat to ensure it cooks evenly.
It can be useful to check the beef with a temperature probe if you have one available. Mid 50s centigrade will be ideal for this cut.
Once you’re happy with how the pincanha is cooked set aside to rest for 10 minutes.
Carve and season the inside of the slices of beef with salt and pepper. Plate up alongside a generous dollop of the celeriac mash, sharply dressed watercress and the pickled walnuts.