Heat olive oil in a frying pan over medium heat and sauté the onion and add the garlic.
Cool the onion mixture and add to a large bowl with the pork, breadcrumbs, salt, pepper, sage and fennel seeds.
With clean hands, mix until well combined.
Divide the mix into 3 equal portions and refrigerate while preparing the pastry.
Remove the puff pastry from the freezer.
You will need one and a half pastry sheets — assuming that the pastry sheets are 25cm x 25cm. Cut the pastry sheets in half. Each half will make two sausage rolls once you fill, roll and cut in half lengthways.
Once you have your pastry cut and ready, put the rest of your puff pastry back in the freezer.
Place one log of the filling in the centre of the pastry and pat out to fill the length of the pastry.
Lightly beat the egg and milk together. With a pastry brush, brush one end of each sheet of the pastry.
Fold the pastry from both front and back and create a light seam in the middle. Lightly press the seam together.
Cut the roll in half, making two 12.5 cm long sausage rolls.
Place the rolls seam-side down on a baking tray lined with baking paper. Brush the top of the rolls with the egg wash.
Refrigerate while you make the remaining rolls.
Preheat the oven to 200˚C and bake the rolls until they are golden brown and cooked through (internal temperature should be 71˚C). This should take around 20 to 30 minutes depending on the oven. I like to rotate the tray half way through to help get an even golden brown crust.
Serve with tomato sauce.