01 -
Heat olive oil in a skillet over medium heat. Add sliced onions and a pinch of salt, then cook gently, stirring occasionally, until deeply golden and caramelized, approximately 15 to 20 minutes. Allow to cool slightly.
02 -
In a bowl, mix ground sausage, caramelized onions, thyme, salt, and black pepper. Incorporate Gruyère cheese and Dijon mustard if desired.
03 -
Preheat oven to 200°C. Line a baking sheet with parchment paper.
04 -
On a lightly floured surface, unroll the puff pastry and cut into rectangles roughly 10 by 15 centimetres. Place a portion of filling along one side of each rectangle, fold the pastry over, and seal the edges with a fork.
05 -
Transfer sausage rolls to the prepared baking sheet. Brush the tops with beaten egg. Bake for 20 to 25 minutes until the pastry is crisp and golden brown. Allow to cool slightly before serving.