Preheat your oven to 200°C (400°F).

In a skillet, sauté the chopped onion until soft and translucent.

In a mixing bowl, combine the sausage meat with sautéed onion, sage, garlic powder, black pepper, and salt.

Roll out the puff pastry on a floured surface and cut it into rectangles.

Place a generous amount of filling in the center of each rectangle.

Fold the pastry over the filling and seal the edges with a fork.

Brush the tops of the sausage rolls with the beaten egg.

Place the rolls on a baking sheet lined with parchment paper.

Bake in the preheated oven for 25-30 minutes or until golden brown.

Let cool slightly before serving.