1. Make a quick béchamel sauce:
2. Mix the cornstarch with some of the cold milk.
3. Boil the milk in the microwave, variable duration depending on the power of the microwave
4. Pour the mix milk / corn starch into the boiling milk
5. Mix and heat for 1 minute at full power in the microwave. If need be put back to heat.
6. Season and add the grated cheese and possibly the oil and let cool.
7. Roll out the puff pastry to make 4 rectangles about 2 to 3 mm thick.
8. Garnish the cooled béchamel over half a rectangle leaving a small border.
9. Pass a little water on this border.
10. Fold the rectangle over the stuffing and glue by pressing the edges with a fork.
11. Brown with the egg, being careful not to run it on the edges; the puffing would swell less, make some cuts.
12. Bake in preheated oven (200 to 225 ° C) for 20 to 30 minutes.