Stir together vinaigrette and mustard; set aside 1/2 cup.
Combine flank steak and remaining vinaigrette mixture in shallow dish or resealable plastic bag. Cover or seal and marinate in refrigerator at least 2 hours.
Drain steak; discard marinade. Grill covered over medium-high heat 6 to 8 minutes on each side or to desired doneness. Slice diagonally across the grain. Toss over mixed greens with potatoes, green beans, tomatoes, eggs and olives; drizzle with reserved vinaigrette mixture.
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