Place potatoes in microwave-safe dish; cover with plastic wrap, venting one corner. Microwave on HIGH 7 minutes (Note: Microwave cooking time is based on a 700-watt microwave. Adjust time to your own microwave), or until just tender; set aside.
Cut steak lengthwise into thirds, then crosswise into ¼-inch thick slices. Heat 1 tablespoon oil in large nonstick skillet over medium-heat until hot. Add ½ steak; cook and stir 2 minutes or until steak loses pink color. Using slotted spoon, remove to large bowl; repeat with remaining oil and steak.
Add Worcestershire sauce; toss to combine. Add potatoes to skillet; cook and stir until most liquid is absorbed. Add steak, cheese and onions; mix lightly just until cheese melts. Season with salt and pepper, if desired.
Spoon ½ cup potato mixture onto center of each tortilla. Fold bottom edge up over filling; fold right and left sides to center, overlapping edges; fold top edge over to enclose filling. Serve with steak sauce, if desired.