To make the marinade, combine the port and pepper and mix well. Coat the fillet with the mixture, cover and leave to marinate in the fridge for approx. 3 hrs. Remove the meat from the fridge approx. 1 hr. prior to cooking. Preheat the oven to 80°C. Warm the platter and plates. Pat the meat dry, season with salt. Heat the clarified butter in a frying pan. Brown the meat all over for approx. 10 mins. Put the pan to one side. Place the meat on the warmed platter. Insert the meat thermometer in the thickest part.
Cook at a low temperature for approx. 1 1/2 hrs. in the centre of a preheated oven. The core temperature should be 60-65°C. The meat can then be kept warm at 60 °C for up to 1 hr.
Mix the rhubarb with the icing sugar, cover and leave to stand for approx. 30 mins. Discard the juice produced. Place the sugar and butter in the frying pan and bring to the boil without stirring. Reduce the heat and simmer, shaking the pan occasionally, until a light brown caramel has formed. Add the onions and ginger and sauté for approx. 2 mins. Pour in the stock, bring to the boil, simmer over a medium heat for approx. 5 mins. Add the rhubarb, sprinkle over the sugar, cover and simmer for approx. 5 mins. until almost soft, season.