1. A) Make a quick béchamel sauce:
2. 1- Dissolve the cornstarch in a little milk.
3. 2- Bring the remaining milk to a boil.
4. 3- When it is very hot, pour in the dissolved cornstarch and stir constantly with a whisk or wooden spatula until the mixture thickens.
5. 4- Remove from heat as soon as it boils; add the grated gruyère cheese and optionally butter, and let it cool.
6. B) Roll out the puff pastry to obtain 4 rectangles of approximately 2 to 3 mm thick.
7. C) Fill one half of each rectangle with the cooled béchamel sauce, leaving a small border.
8. Moisten this border with a little water.
9. Fold the rectangle over the filling and press the edges together with a fork to seal.
10. Brush with beaten egg, being careful not to let it drip onto the edges; this would prevent the puff pastry from rising as much. Make a few cuts.
11. Bake in a preheated oven (200 to 225°C) for 20 to 30 minutes.