1. Prepare a quick béchamel sauce:
2. Dissolve the cornstarch in a portion of the cold milk.
3. Boil the milk in the microwave, duration varies depending on the microwave's power.
4. Pour the milk/cornstarch mixture into the boiling milk.
5. Mix and heat for 1 minute at full power in the microwave. If necessary, heat again.
6. Season and add the grated gruyère cheese and optionally the oil, then let it cool.
7. Roll out the puff pastry to obtain 4 rectangles of approximately 2 to 3 mm thick.
8. Fill one half of each rectangle with the cooled béchamel, leaving a small border.
9. Moisten the border with a little water.
10. Fold the rectangle over the filling and press the edges with a fork to seal.
11. Brush with beaten egg, being careful not to let it drip onto the edges; this would prevent the puff pastry from rising. Make a few cuts.
12. Bake in a preheated oven (200 to 225°C) for 20 to 30 minutes.