Preheat the oven to 400°F (200°C).

In a large skillet, over medium heat, add a splash of olive oil. Once hot, sauté the chopped onions and minced garlic until soft and fragrant, about 5 minutes.

In a large bowl, mix the sautéed onion, sausage meat, Dijon mustard, salt, pepper, and any fresh herbs using your hands.

Roll out your puff pastry sheets on a floured surface and cut them into rectangles, roughly 4 inches by 6 inches.

Place a generous spoonful of the sausage mixture at one end of each rectangle, tuck in the edges, and roll up like a burrito. Seal the edges with a fork.

Place the rolls on a baking sheet lined with parchment paper and brush them with the beaten egg.

Bake the rolls for about 20-25 minutes, or until golden and puffed up.

Let them cool for a few minutes before serving.