At 1 st. a spoonful of vegetable oil, fry the onion until golden brown. Transfer to a plate.
In the same skillet, on one spoonful of oil, fry the steak on both sides until brown. Then transfer the steak to a plank, cut into thin strips.
Brush both slices of bread with mustard on one side only. Put cheese, meat, and onions on one of the slices (on the side greased with mustard), cover with the second slice, clean side up.
Place this sandwich on a press grill, cover, and cook until the bread is golden brown, 1-2 minutes.