For the terrine, double up a 30x30cm/12x12in piece of cling film and use it to line a 15x15x5.5cm/6x6x2¼in plastic container, leaving an overhang on all sides to wrap the terrine later.
Prepare the leeks by peeling off the two outer layers (they will be too fibrous). Divide each leek into two 15cm/6in lengths. In a large saucepan on a high heat, bring 3 litres/5¼ pints water and the salt to a rolling boil. Cook the leeks for 20 minutes, until completely tender. Using a slotted spoon, lift them onto a tray and leave to cool completely.
For the vinaigrette, in a bowl, whisk all the ingredients with 1 teaspoon water using a hand-held blender. Taste and adjust the seasoning if necessary.
To build the terrine, layer the leeks tightly side by side in the lined container (eight pieces per layer) and brush with a little of the vinaigrette. Cover with a few of the truffle slices, if using, and press down. Repeat for the second layer, alternating the white and greener parts of the leeks. Continue until you have three layers of leeks, then fold over the overhanging cling film, place a second 15x15x5.5cm/6x6x2¼in plastic container on top and fill it with 1kg/2lb 4oz of weight to compress the terrine (you could use some tins of beans, for example). Leave for 12 hours, or overnight. Store the remaining vinaigrette in the fridge for serving.
To serve, lift the terrine from its container onto a chopping board, keeping the cling film wrapped around (this will keep the structure of the terrine when you slice it). Using a carving knife, slice the terrine into eight even slices, approximately 2cm/¾in thick. Carefully peel off the cling film and use a palette knife to transfer a slice to the centre of each plate.
Whisk the chives into the reserved vinaigrette, then brush this over the top of each slice of terrine. If serving with the Jerusalem artichokes, dress them with the remaining vinaigrette and scatter them around the terrine. Finish with some fine shavings of fresh truffle, if using, and winter leaves.