In a large nonstick skillet, cook beef over medium heat for 5-7 minutes on each side or until meat reaches desired doneness (for medium-rare, a thermometer should read 135°; medium, 140°; medium-well, 145°). Cool for 10 minutes. Refrigerate until chilled.
Thinly slice the beef; place in a small bowl. Add the onion, salt and pepper; toss to combine. Divide lettuce between two salad plates; top with beef mixture, green pepper, pimientos and olives. Combine sour cream and lemon juice; serve with salad.