Trim fat and cut steak into 1/2-inch (1 cm) wide strips. Whisk together soy sauce, cornstarch, sugar, sesame oil and hot pepper flakes; toss with steak. Refrigerate for at least 30 minutes or for up to 4 hours.
In large skillet, heat vegetable oil over medium-high heat; stir-fry red onion and garlic for about 2 minutes or until fragrant. Remove beef from marinade (discard any leftover marinade) and add beef to skillet; stir-fry for about 6 minutes or until browned and cooked to medium. Transfer to plate.
Add nectarines to pan; stir-fry for 5 to 7 minutes or until warmed through. Add chicken broth and return reserved beef mixture along with any accumulated juices to pan. Bring to simmer; cook for about 4 minutes or until sauce is thickened. Sprinkle with green onion. Serve over rice.
Tip: Serve with lime wedges if desired.