Clean and quarter the mushrooms. Salt and pepper the beef rump steaks.
Heat the oil in a pan, fry the steaks on each side for 3 to 4 minutes over high heat, remove and keep warm.
Add butter, mushrooms and onions to the pan and fry for 2 minutes. Deglaze with cognac, pour in the whipped cream and cook for 1 minute. Stir in the cucumber cubes and mustard, season with salt and pepper.