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 the sliced onions and salt, then cook for 25 to 30 minutes, stirring often, until soft, golden, and caramelized.

3. Stir in garlic, thyme, Worcestershire sauce, Dijon mustard, and black pepper. Cook for 1 minute, then remove from heat and let the onion mixture cool.

4. In a mixing bowl, combine the ground sausage, cooled onion mixture, and shredded Gruyere cheese. Mix gently until evenly combined.

5. Unroll the cold puff pastry on a lightly floured surface and cut it into long rectangles.

6. Spoon the sausage filling in a long line down the center of each pastry rectangle. Fold the pastry over the filling and press the seam to seal.

7. Place the rolls seam-side down, then cut them into smaller pieces. Transfer to the prepared baking sheet.

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

9. Bake for 22 to 26 minutes, or until the pastry is puffed and golden and the sausage filling is fully cooked.

10. Let the sausage rolls rest for 5 minutes, then garnish with parsley and serve warm.