Wash the potatoes under cold running water, place in a steamer and cook until the potatoes are tender. Remove from the steamer and allow to cool slightly before peeling. Slice the potatoes into 1cm thick slices. Place the mushroom duxelle into a bowl and add the truffle oil, parsley and thyme. Mix together and season to taste with salt and freshly ground pepper.
Line a terrine mould 32cm in length, 8cm wide and 7cm deep with cling-film, with a 1 inch overhang. Trim the potatoes on all sides into even blocks so that they fit neatly into the terrine. Lightly season the potatoes with salt and freshly ground pepper and lay the first layer into the base of the terrine so that the entire surface is covered with potato. Brush the potato with clarified butter and then spoon a 1cm deep layer of the mushroom duxelle over the potato and smooth out with a palette knife. Lay the next layer of potato and continue to build the terrine finishing with a layer of potato. The finished terrine will have 3 layers of mushroom duxelle and 4 layers of potato. Cover the terrine with the overhang of cling-film. The terrine should be lightly pressed so that the various layers hold together. Cut a piece of cardboard the same size as the terrine and cover with first with aluminum foil followed by cling-film. Place it on top of the terrine and place a 2kg weight evenly on top of the terrine. Refrigerate for 12 hours.
Remove the weights and cardboard from the terrine and remove the cling-film from the top of the terrine. Run a sharp knife around the inside of the mould. Invert the terrine onto the cut-out cardboard. Using a sharp knife cut a 1cm thick slice of the terrine, brush with the olive oil and lightly season with salt and freshly ground pepper. Allow to stand for 10 - 15 minutes to reach room temperature. Place the terrine in the centre of a plate and spoon the mushroom a la Grecque around it. Spoon a little more of the a la Grecque over the mushrooms and garnish with the chive tips, parsley and mache leaves.