First, make the filling as follows. Clean and finely chop the onions. Dice the ham and 3.5 oz (100 g) of the cheese finely.
Prepare the Bechamel Sauce by putting the butter to melt in a small pot. To it add the flour, while stirring energetically with a spoon.
Once braised, pour in the cold milk all at once and stir until it comes to a boil and becomes a nice homogeneous sauce. Add salt, black pepper and nutmeg to taste.
Lay out the puff pastry onto a counter or table, cut into identical squares or rectangles.
I ended up with 8 rectangles. Smear the edges of the rectangles with beaten egg. Put a little ham, cheese and a spoon of the cooled Bechamel Sauce in the middle of each.
Fold 1 end of the rectangle over the filling. Press the turnovers with your hands or with a spoon to seal closed.
Transfer to a tray covered with baking paper, smear with beaten egg on top and sprinkle on the remaining grated cheese. Bake in a preheated 392°F (200 °C) oven.