Preheat oven to 220ºC, (200ºC for fan assisted), 425ºF, Gas Mark 7.
Unroll pastry sheet and cut in half across the width to give 2 oblong pieces of pastry.
Spread the centre of each pastry piece with the mustard, leaving a border on each long side.
Divide the sausage meat into half and place one quarter slightly off centre down the length of one of the pastry oblongs, over the mustard and in a ‘roll’ form.
Brush one border lightly with beaten egg and fold over the pastry to cover the sausagemeat and form a side seal along the edge of the roll. With a sharp knife knock up the edges, cut the roll into 3 or 4 equal sections and brush with beaten egg. Make 2-3 slashes in the top, repeat with remaining ingredients and place the sausage rolls on a baking sheet. Scatter with chopped sage if desired.
Bake for approx. 15 minutes until pastry is risen and golden brown, serve hot or cold.