Start by thinly slicing the flank steak against the grain and place it into a bowl. Sprinkle cornstarch over the beef, tossing gently until each piece is fully coated. Set aside.

Bring a large pot of salted water to a boil. Add the egg noodles or lo mein noodles, cooking until tender yet still slightly firm, about 3–5 minutes. Drain and set aside.

Heat vegetable oil in a large skillet or wok over high heat. Add half of the coated beef and stir-fry for 2–3 minutes until browned. Remove and repeat with remaining beef.

In the same skillet, add more oil if needed, and stir-fry garlic and ginger for 30 seconds. Then add onion, bell pepper, broccoli, and carrots, stir-frying for 3–4 minutes until vibrant.

Whisk together soy sauce, mirin, brown sugar, sesame oil, and cornstarch slurry until combined. Pour into skillet with vegetables and simmer for 2–3 minutes until thickened.

Reduce heat to medium and return cooked beef and noodles to skillet. Toss gently until everything is evenly coated with the teriyaki sauce.

Serve the Beef Teriyaki Noodles in bowls garnished with green onions. Enjoy hot!