In a bowl, mix the lukewarm milk, sugar and yeast, let the yeast activate for 10 minutes.
Add the sifted flour, salt, mix, then knead for 10 minutes and shape into a ball, cover with cling film and let it double in volume.
For the filling, mix:
Grated mozzarella, grated cheese and chopped onion.
Spread the dough on the work surface sprinkled with flour, roll it into a rectangular shape with a thickness of 2-3 mm, grease with melted butter and sprinkle with filling over the entire surface.
Roll out lengthwise and divide the dough into 12-14 pieces.
Transfer the pieces to a buttered tray and let them double in volume.
Brush with beaten egg and bake for 20-25 or even 30 minutes at 180 °C.
