Mix broth, soy sauce, vinegar, oyster sauce, cornstarch, ginger, five-spice powder, sugar and red-pepper flakes in small bowl.
Heat oil in large skillet or wok over medium-high heat. Add carrots; cook, covered, stirring occasionally, 5 minutes. If using broccoli, see Note below. Add beef, garlic, salt, black pepper; stir-fry, until beef is no longer pink, 5 minutes. Add sweet red pepper and scallion; stir-fry until softened, 3 minutes.
Rewhisk sauce; add to skillet; cook until thickened, 3 minutes. Serve with cooked hot rice, if desired.