Stir the flour with the salt and the sugar. Make a well in the middle, pour in the warm water and knead well.
Add the yeast with another 2 tablespoons of warm water and keep kneading until everything is worked in.
Stretch the dough into a rectangle, put it on a tray, cover it with plastic wrap and refrigerate for 1 hour. This is a chill, not a rise: cold dough is what makes the butter possible to laminate.
Soften the butter and flatten it with a rolling pin into a sheet.
Take the dough out, put it on a floured surface and roll it out.
Cut it into a rectangle and lay the sheet of butter on top.
Fold the dough over the butter and roll it out again. Repeat this three times, chilling it briefly if the butter starts to come through, until you have layers like puff pastry.
Roll the dough out and cut it into triangles.
Lay two sheets of mortadella and some thin pieces of cheese on each triangle.
Roll each one up starting from the wide end.
Brush them with the beaten egg.
Leave them on the tray in a warm place, covered with a cloth, until they have visibly risen and feel airy when you touch them. This takes about an hour and a half and it is the step that decides whether you get a croissant or a bread roll.
Bake at 200°C (390°F) for 15 minutes.