Lightly coat 12" skillet with cooking spray, and heat skillet until hot over high heat.
Add onions and cook, stirring onions for about 4 minutes or until the onions are browned.
Remove onions from skillet, set aside, then use paper towel to wipe out skillet.
Coat same skillet with cooking spray and heat until hot. Add beef and cook for about 10 to 13 minutes, turning meat once, for medium rare to medium. Remove beef from heat and transfer to cutting board. Let stand for about 3 minutes before cutting.
Return skillet back to cooktop and heat over high until hot. Add onion, water, Worcestershire sauce, and sugar. Cook the mixture for 30 to 45 seconds or until the liquid has mostly evaporated.
Slice beef diagonally into thin pieces. Serve sliced beef with caramelized onions.