Make the sauce. Combine orange juice, soy sauce, garlic, ginger, crushed red pepper flakes, sugar, sesame oil, cornstarch, five-spice powder, and orange zest. Stir well and set aside.
Slice the beef. Cut steak against the grain into thin strips (roughly 1/4 inch). Toss beef with a small pinch of salt.
Cook the beef in 2 batches. Heat about 2 teaspoons olive oil in large non-stick frying pan over medium-high heat. Add half the beef strips and cook about 3 - 4 minutes, stirring frequently, until beef is done and starting to brown. Remove cooked beef to clean plate. Add another 2 teaspoons olive oil to pan and fry remaining beef for about 3 - 4 minutes minutes until it's done and starting to brown. Remove cooked beef and set aside with first batch.
Cook the vegetables. Heat remaining 2 teaspoons olive oil in the pan over medium-high heat. Add the vegetables and cook for about 3 to 4 minutes, stirring frequently, until vegetables are softened. (Cover the pan for about 2 minutes if using broccoli.)
Give the sauce a good stir and pour into the pan with vegetables. Add the beef back to the pan. Cook everything for another 2 minutes, stirring frequently, until sauce boils and starts to thicken.
Serve over steamed white rice.