1. Preheat the oven to 400°F and line a baking sheet with parchment paper.

2. Heat butter and olive oil in a skillet over medium-low heat. Add sliced onions and a pinch of salt, then cook for 25 to 30 minutes until soft, golden, and caramelized.

3. Stir in garlic and thyme during the last 2 minutes of cooking, then remove the onions from the heat and let them cool for 10 to 15 minutes.

4. In a bowl, combine ground sausage, cooled onions, Worcestershire sauce, Dijon mustard, Gruyère cheese, breadcrumbs, black pepper, and salt. Mix gently until combined.

5. Roll out the puff pastry and cut it into two long rectangles.

6. Divide the sausage mixture into two logs and place one log down the center of each pastry rectangle.

7. Brush one long edge with beaten egg, fold the pastry over the filling, and seal the edge firmly. Place seam side down.

8. Cut each roll into smaller pieces and arrange them on the prepared baking sheet. Chill for 15 minutes.

9. Brush the tops with beaten egg and sprinkle with sesame seeds or poppy seeds.

10. Bake for 22 to 26 minutes, or until the pastry is deep golden brown and the sausage filling is fully cooked.

11. Let the rolls rest for 5 minutes before serving warm.