Preheat the oven: preheat the oven to 180°C / 350-375°F / Gas Mark 4-5. **Or do this after you have made the caramelised onions**.
Onions: use a sharp knife and cutting board to peel the onions then thinly slice them. Once done, use your fingers to separate the onions into individual rings.
Caramelise the onions: place a large sauce pan onto the stove top, add the olive oil and turn the heat to high. Add the sliced onions and cook stirring occasionally for 10 minutes. Turn the heat down during this time if the onions or pan look like they are starting to brown too much. Next, if you haven't already, turn the heat down to low and then add the butter. Stir this in with a wooden spoon and let it melt. Next add the balsamic vinegar and brown sugar. Stir the onions every now and then and let them cook and caramelise for 10 to 15 minutes. Once the onions have a jam like appearance and texture turn the heat off.
Combine: let the onions cook for a couple of minutes. Once they have add them to a food processor and blitz for 10 to 15 seconds. Next add the pork mince, panko breadcrumbs, dried thyme, salt, pepper and egg. Blitz these to combine and then remove the lid and blade from the food processor and transfer the filling to a large mixing bowl.
Egg: crack the remaining egg into a small glass and use a fork to whisk then set aside.
Roll: place a thawed sheet of puff pastry onto a clean, dry chopping board. Starting at one end, transfer a sausage size amount of the pork and caramelised onion filling onto the pastry and use your fingers to shape this into a long log like shape. Next roll the pastry around the filling leaving an overlap where the start and finish ends, Then use the whisked egg and a basting brush to seal the pastry. Next use a knife to cut the remaining pastry off and the same knife to cut the long rolled shape into 6 equal pieces. Transfer these individual sausage rolls onto a silicon mat on a baking tray.
Cook: baste each of the individual sausage rolls with the whisked egg. Then transfer the baking tray into the oven on the middle shelf. Cook the sausage rolls for 25 to 30 minutes.
Cool: use oven mitts to remove the cooked sausage rolls from the oven and place them onto a cooling rack. Let them cool for a couple of minutes and then use a pair of tongs to remove them from the baking tray and place them directly onto the cooling rack.
To serve: serve the sausage rolls warm/hot with tomato sauce, barbecue sauce, plum sauce, sweet and sour sauce or your favorite sauce!