Cut steak into 1/4-inch strips. Add steak to soy marinade; toss to coat. Cover and refrigerate 15 minutes.
Add 1 teaspoon oil to skillet or wok; heat over medium-high heat until shimmering.
Add steak, garlic, ginger and pepper flakes to skillet or wok. Stir-fry 1-2 minutes or until outside surface of steak is no longer pink; remove from skillet.
Heat remaining oil in same skillet until shimmering. Add vegetables; stir-fry 3-4 minutes or until crisp-tender.
Return steak and add broth. Bring to a boil. Boil for 1 minute, stirring gently.