Pre-heat oven to 200°C/390°F or 180° Fan
Place frozen pastry sheets on work bench and leave aside while you make the filling (if you leave the plastic sheet underneath the pastry, it will be easier to roll it up.
In a small pan, melt the butter and sauté the onion and garlic until soft and just lightly golden (about 5 minutes)
To a mixing bowl, add the sausage mince, onion and garlic, parsley, salt, pepper and egg yolk.
Wearing disposable gloves, mix the ingredients by hand, then either divide into 6 balls or place the mixture into a large piping bag
Shape each ball into a sausage, placing it lengthways across each of the pastry rectangles. Alternatively, pipe a sausages onto each rectangle
Using a pastry brush, use the extra egg to brush along the long edge of each pastry sheet. This will seal the pastry once it's rolled up.
Tightly roll the pastry over the sausage mixture, finishing at the edge that's been brushed with the egg. Press down to seal the edges, then use a serrated knife to cut each long roll into 6 smaller sausage rolls ( or cut to desired length, for larger sausage rolls). You can use the plastic sheet under the pastry to help you form the rolls -just peel it away once you've got the roll started.
Place the sausage rolls onto a lined baking sheet and brush the rolls with the egg and sprinkle with sesame seeds.
Bake for 30-35 minutes, until golden.