Grate the cheese roughly. Cut the cucumbers lengthwise into slices. Peel and halve the onion and cut into strips.
Dab meat dry. Heat oil in a pan (with lid). Fry the steaks in it at high heat for 1 minute on each side. Fry over medium heat for about 2 minutes on each side. Season with salt and pepper and remove.
Fry the onion in the frying fat for about 2 minutes. Cut meat into strips. Add barbecue sauce to the pan. Spread meat on top and sprinkle with cheese. Put the lid on the pan and leave it on the switched off hotplate until the cheese has melted.
In the meantime, toast the bread bit by bit in the toaster. Cover 4 slices of bread with cucumber. Spread steak strips with onions and sauce on the slices and cover with the remaining bread slices.