1 pound beef top round or top sirloin steak, cut 3/4 inch thick or flank steak
1/2 cup prepared Italian dressing
Salt and pepper
2 large bell peppers, any color, cut into 1/8 inch strips
1 onion, cut into 1/8 inch wedges
Cut beef steak lengthwise in half, then crosswise into 1/8-inch thick strips. Place beef and 1/3 cup Italian dressing in food-safe plastic bag; turn beef to coat. Reserve remaining dressing for vegetables. Close bag securely and marinate in refrigerator 30 minutes to 2 hours.
Remove beef from marinade; discard marinade. Heat large nonstick skillet over medium-high heat until hot. Add 1/2 of beef; stir-fry 1 to 2 minutes or until outside surface of beef is no longer pink. (Do not overcook.) Remove from skillet. Repeat with remaining beef. Season beef with salt and pepper; keep warm. Wipe skillet with paper towel.
Heat remaining Italian dressing in same skillet until hot. Add peppers and onion. Stir-fry 5 to 6 minutes or until vegetables are crisp-tender.
Return beef and juices to skillet. Cook and stir 1 to 2 minutes or until heated through.