Melt oil and butter in pan. Add beef to pan. When beef is browned, season with salt and papper and remove it from the pan.
Add tomatoes, onion and mushrooms and sauté until done. Add lemon juice, mustard and cucumbers and simmer for five more minutes. Add sour cream; do not let it come to a simmer or boil or the sour cream will curdle.
Serve over cooked egg noodles, rice or baked potatoes.
