Peel the onions, cut them into thin strips and fry them in a little fat, stirring constantly until golden brown. Set aside.
Cut the pork cutlets diagonally for stuffing, unfold and spread with a little mustard.
Spread the onions evenly over them, season a little. Fold the cutlets and fix them with toothpicks.
Beat the egg, season.
Turn the stuffed cutlets first in the egg, then in breadcrumbs and slowly bake in the oil-butter mixture on both sides until golden brown.
Drain on paper towels and serve.