Stir the flour with the salt and sugar. Open a hole in the middle and add the warm water. Knead very well.
Add the yeast plus another 2 tablespoons of warm water and continue kneading all the ingredients well.
Stretch the dough and give it a rectangular shape, put it on a tray, cover it with plastic wrap and refrigerate for 1 hour.
Soften the butter, and with the help of a rolling pin flatten it.
After the dough's time, place it on a floured surface and shape it with the rolling pin.
Cut the dough into a rectangle and place a portion of butter on top.
Fold the dough and in this way join the butter with the dough. Repeat this process 3 times until you have a perfect dough similar to puff pastry.
Shape the dough and cut it into triangles.
Add two sheets of mortadella and cheese in thin pieces.
Roll the dough starting from the thickest part from one of the corners.
Once rolled, brush the croissant with the beaten egg.
Place them in the oven at a temperature of 200°C for 15 minutes. And it's ready.