Heat the margarine in the pan, add the onion cut into cubes and sauté until light brown.
Remove fat and fascia (soft tissue from connective tissue) from the meat and cut the fillet into thin, elongated strips. Season with salt. Then add the strips of meat to the onions in the pan and fry for about 3 minutes. Deglaze with cream.
Add the sauce, Worcestershire sauce and diced cucumber. Simmer the whole thing for about 8 minutes. If the sauce is still a little too thin, thicken a little with a sauce thickener (brown). Season to taste with salt and fresh pepper.