Preheat oven to 200°C bake. Line a large baking with baking paper
Squeeze the sausage meat out of the sausage casings and place in a bowl. Add the onion, parsley and tomato sauce and mix well.
Cut each pastry sheet in half, to form 6 pastry rectangles.
Lay one-sixth of the sausage mixture in a line along with one of the long edges of a pastry rectangle. Brush the long edges of the pastry with a little water and roll up to enclose the filling. Repeat to form 6 long rolls. Lift the rolls onto the prepared baking tray.
Mix together the egg yolk and water and brush over each roll. Use a sharp knife to make shallow slits across the pastry, marking the eventual size of the sausage rolls. You will cut along these once they are cooked.
Bake for 15-20 minutes, until golden brown and cooked through
Cool slightly on the baking tray, then slide the rolls onto a cutting board and cut up with a sharp knife. serve hot or cold.