Coarsely grate the cheese. Cut the cucumbers lengthwise into slices. Peel onion and cut into strips.
Dab meat dry and season with salt. Heat the oil in a pan and fry the steaks for about 3 minutes on each side. Season steaks with pepper, take them out, wrap them in aluminium foil and let them rest.
Sauté the onion strips in the frying fat for about 2 minutes. Cut meat into strips. Add the BBQ sauce to the pan. Spread the meat on top and sprinkle with cheese. Melt the cheese briefly in the closed pan on a switched-off hotplate. Toast the bread in a toaster in portions.
Cover 4 slices of bread with cucumber. Spread steak strips with onions and sauce on the slices and spread the remaining bread slices on top.