Preheat oven to 180°C or Gas Mark 4. Cut the Jerusalem artichokes into thin wedges, putting them in a bowl of cold water with a lemon juice to stop them from browning. Cook in boiling salted water for 8 minutes, until tender, then drain. Heat the oil in a skillet over medium heat. Add the pork cubes and fry for a few minutes on each side until browned. Remove to a plate. In the same skillet, add the butter, leeks and sage. Cook for a few minutes to soften the leeks. Add the flour and cook, stirring for a couple of minutes. Gradually add the milk, together with the stock cube, stirring continually, until the sauce has started to thicken. Let it bubble for a few minutes.  Stir in the Jerusalem artichokes, pork, mustard and thyme leaves. Season with salt and pepper and transfer to a pie dish, allowing the mixture to cool. Lightly brush one sheet of filo pastry with some of the melted butter and place it flat on top of the pie mixture, trimming away any excess. Working with a sheet of filo at a time, brush it with butter, cut it into 5-6cm strips and layer them over the pie to make a ruffled pattern. Keep going until you have created at least a couple of layers of ruffled pastry, tucking it in around the edges. Bake for 45 minutes, until the top is golden brown.