Step 1: Caramelize the onions

1. Heat the olive oil in a large skillet over medium heat. Add the sliced onions and cook, stirring occasionally, for about 15 minutes until softened.
2. Sprinkle the sugar over the onions and continue cooking for another 5-10 minutes until golden and caramelized.
3. Stir in the balsamic vinegar and thyme, then cook for another minute. Remove from heat and let cool.

Step 2: Prepare the filling

1. In a large bowl, mix the sausage meat, breadcrumbs, Dijon mustard, and the cooled caramelized onions. Season with salt and pepper.

Step 3: Assemble the sausage rolls

1. Preheat your oven to 400°F (200°C). Line a baking sheet with parchment paper.
2. Roll out the puff pastry sheet on a lightly floured surface. Cut it in half lengthwise to create two long strips.
3. Divide the sausage mixture into two portions. Shape each portion into a long log and place it along the center of each pastry strip.
4. Brush the edges of the pastry with beaten egg. Fold the pastry over the sausage, pressing the edges to seal. Place the rolls seam-side down on the baking sheet.

Step 4: Cut and garnish

1. Cut each roll into smaller pieces (4-6 pieces per roll, depending on your preferred size). Brush the tops with the remaining egg wash.
2. Sprinkle with sesame seeds or poppy seeds, if desired.

Step 5: Bake the sausage rolls

1. Bake in the preheated oven for 20-25 minutes, or until the pastry is golden and puffed, and the sausage filling is cooked through.

Step 6: Serve

1. Let the sausage rolls cool slightly before serving. Serve warm with mustard, chutney, or a dipping sauce of your choice.