Pre-heat your oven to 180c. Place the diced potatoes in a pan of cold, salted water and bring to the boil. Reduce the heat and simmer for 15 minutes, or until a fork slides easily through the potatoes.
While the potatoes are cooking, spray a frying pan with low calorie cooking spray and sauté the onions over a medium to low heat, until they are soft and a golden brown colour. This can take up to 15 minutes. Don't be persuaded to increase the heat to speed this up as the slow cooking caramelises the onions and develops the flavour.
When the potatoes are cooked, drain and place in a bowl. Roughly mash them with a fork - it's fine to have some pieces remaining.
Add the grated cheese (reserving a handful to top the pie), onions, mustard and garlic granules and the Henderson’s Relish.
Beat the egg, add to the potato and cheese mixture and mix thoroughly.
Spray a deep pie dish with low calorie cooking spray. A metal dish is best for this, as it helps to evenly cook the bottom.
Spray each sheet of filo pastry with low calorie cooking spray, and line the pie dish, leaving 3-4 cm over the edge of the dish.
Spread the filling evenly into the pie dish.
Scrunch up the filo around the edge and sprinkle the remaining cheese over the potato mixture.
Place in the oven and cook for 30-35 minutes, until golden and firm.
Cut into 6 pieces and serve.