To prepare the fillet
Remove the meat from the fridge approx. 1 hr. prior to cooking. Preheat the oven to 80°C, warm the platter and plates.
Roughly chop the mushrooms, slice the onions. Finely chop the garlic and sage. Heat the oil in a frying pan. Fry the mushrooms for approx. 10 mins. until all of the liquid has evaporated. Reduce the heat. Add the onions, garlic and sage, cover and sauté for approx. 10 mins. Add the cognac, reduce completely, season with salt. Leave the filling to cool.
Cut lengthwise into the middle of the meat using a long, thin knife, work the knife back and forth a few times until the incision is approx. 4 cm wide. Use the end of a wooden spoon to make the pocket bigger if necessary. Stuff the filling into the pocket using the spoon.
To brown the meat
Heat the oil in a frying pan. Season the meat, brown all over for approx. 5 mins., only turning when a crust has formed, remove. Place the meat on the warmed platter. Put the pan to one side.
Approx. 1 1/4 hrs. in the centre of a preheated oven.
Celeriac & pear puree
Peel the potatoes and celeriac, cut into pieces. Peel and core the pears, cut into pieces. Place a steamer basket in a wide pan, fill with water to just below the bottom of the basket. Add the vegetables and pears, cover and cook for approx. 30 mins. until soft.
Remove the steamer basket containing the vegetables and pears, pass the vegetables and pears through a food mill and back into the pan. Stir in the butter, season. Cover and keep warm.
Pour the cognac into the frying pan, loosen any bits that have stuck to the bottom, reduce almost completely. Pour in the single cream, simmer for approx. 2 mins. Carve the meat across the grain and serve with the celeriac & pear puree and the cream sauce.