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

2. Melt the butter in a skillet over medium heat, then cook the chopped onion for 12 to 15 minutes until soft, golden, and sweet.

3. Stir in the garlic and thyme and cook for 1 minute more, then let the onion mixture cool slightly.

4. In a large bowl, combine the pork sausage, cooked onion mixture, Gruyère cheese, Dijon mustard, salt, and black pepper until evenly mixed.

5. Roll out the puff pastry lightly and cut it into 2 long rectangles.

6. Divide the filling between the pastry strips, shaping it into a log down the center of each strip.

7. Brush one edge of each pastry strip with beaten egg, fold over the filling, and seal the edges.

8. Place the logs seam side down and cut each one into smaller sausage rolls.

9. Arrange the rolls on the prepared baking sheet, brush the tops with egg wash, and score the tops lightly.

10. Bake for 22 to 28 minutes until the pastry is puffed and golden and the filling is cooked through.

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