1. Heat olive oil in a skillet over medium heat. Add sliced onions and sugar; cook, stirring occasionally, until deep golden and caramelized, 20-25 minutes. Season with salt and let cool slightly.

2. In a large bowl combine ground beef, cooled caramelized onions, garlic powder, thyme, Worcestershire sauce, Dijon mustard, shredded cheese, and freshly ground black pepper. Mix until evenly combined.

3. On a lightly floured surface, roll each puff pastry sheet into a rectangle. Cut each sheet into 2-3 long rectangles depending on desired roll size.

4. Spoon an even line of the beef mixture along one long edge of each rectangle. Fold the pastry over the filling and press the edges to seal; crimp with a fork.

5. Place rolls seam-side down on a parchment-lined baking sheet. Brush tops with beaten egg and sprinkle with sesame seeds or herbs if using.

6. Bake in a preheated oven at 400°F (200°C) for 20-25 minutes, or until pastry is puffed and golden brown.

7. Let rest 5 minutes, slice into portions if desired, and serve warm with mustard or caramelized onion dipping sauce.