To make Bechamel Sauce: In a pot, melt the Butter (3 1/2 tablespoon). Remove from heat, and pour in All-Purpose Flour (50 gram). Mix until you get a smooth paste. Allow it to cook over heat for two minutes to cook out the flour taste from the roux, while you stir.
Slowly pour in the Milk (500 milliliter). Add Ground Nutmeg (to taste), Salt and Pepper (to taste). Keep stirring until it starts boiling and gets as thick as you desire. Let cool, then store in a plastic or glass container until needed.
Preheat your oven to 200 degrees C (400 degrees F). Chop up the Mozzarella Cheese (80 gram) into cubes.
Drain and chop the peeled Tomato (14 ounce). Season them with some Salt and Pepper (to taste).
Lay out the Puff Pastry (20). Right in the center of each puff pastry sheet, place 1 tsp of Bechamel sauce, 1 small piece of tomato, and 1 cube of mozzarella cheese.
Roll the puff pastry from one corner to the other into a croissant shape.
In a bowl, beat an Egg (1) with Milk (1 tablespoon), and brush the wash onto each croissant.
Bake at 200 degrees C (400 degrees F) for about 15-20 minutes, or until they get a nice golden-brown color. Remove from the oven and let them cool down a bit. These are best served warm (not hot), but even cold-ish they will rock your boat! Enjoy!