01 - 
                Heat olive oil in a skillet over medium heat. Add sliced onions and a generous pinch of salt. Cook slowly, stirring occasionally, until onions are deeply golden, approximately 15–20 minutes. Allow to cool.
              
              
              
                02 - 
                In a mixing bowl, combine ground sausage, caramelised onions, thyme, salt, and pepper. Add Gruyère cheese and Dijon mustard if using. Mix until fully incorporated.
              
              
              
                03 - 
                Preheat the oven to 200°C. Line a baking sheet with parchment paper.
              
              
              
                04 - 
                On a lightly floured surface, unroll puff pastry sheets and cut into rectangles approximately 10×15 cm. Place a portion of sausage filling along one long edge of each rectangle, fold pastry over, and seal the edges firmly with a fork. Place seam-side down on the prepared tray.
              
              
              
                05 - 
                Brush the tops of the pastry with beaten egg for a glossy finish.
              
              
              
                06 - 
                Bake in the centre of the oven for 20–25 minutes, or until the pastry is deeply golden and crisp. Allow rolls to cool slightly on a rack before serving.