Lightly coat 12" skillet with cooking spray, and heat skillet over medium-high.
Add onions and cook, stirring often, for 4 minutes or until browned.
Remove onions from skillet and set aside.
Coat same skillet lightly with cooking spray and heat to medium-high. Add beef and cook 5-6 minutes per side turning once, until internal temperature reaches 145°F. Remove beef from heat and transfer to cutting board. Let stand 3 minutes.
Return skillet to cooktop and heat to medium-high. Add onions, water, Worcestershire sauce, and sugar. Cook mixture for 30 to 45 seconds until liquid has mostly evaporated.
Slice beef diagonally into thin pieces. Serve with caramelized onions.