Place the Jerusalem artichoke in a bowl, add the salt, pepper and oil, mix well, spread onto a baking tray lined with baking paper.
Bake for approx. 40 mins. in the centre of an oven preheated to 200°C.
Rinse the buckwheat in a sieve, place in a pan with the water and bring to the boil. Cover and remove the pan from the heat, leave the buckwheat to swell for approx. 20 mins. Add the butter and salt before serving.
In a tall container, mix the egg yolk with the mustard, lemon juice and salt. Add the rapeseed oil drop by drop to begin with, then more gradually, stirring constantly to form a thick mayonnaise. Finally, stir in the nut oil.
Season the meat. Heat the oil in a frying pan, brown the meat all over for approx. 5 mins. and transfer to a baking tray, insert the meat thermometer into the thickest part of the meat.
Cook the meat for approx. 20 mins. in the centre of an oven preheated to 200 °C (the core temperature should be approx. 50 °C.
Leave the meat to rest for approx. 5 mins. before carving, serve with the accompaniments and mayonnaise.