Preheat your oven to 200 degrees.
Boil the potatoes in a pan of boiling water for 10 minutes, or until tender. Remove from heat and drain.
Put the potatoes in a bowl and add the cheese, onion, mustard powder, Worcester sauce, and black pepper. Mash together well.
Make two equal sized rectangles from your puff pastry, and lay on a cold, flat surface.
Place half of the cheese and onion filling on one side of each pastry rectangle, and brush one edge with beaten egg. Gently fold both in half to form your rolls, so the egg acts as a seal. Crimp the open edge with a folk, gently add some slits with a knife and brush with beaten egg.
Spritz a baking tray with low calorie cooking spray (to prevent sticking), and place your rolls on the tray. You might find it useful to use a pallet knife to do this.
Bake in the oven for 15 minutes, or until your sausage rolls are piping hot and golden brown. Keep an eye on them so they don't burn!
Serve and enjoy!