Begin by cooking the lo mein noodles according to the package instructions. Drain and set aside.
While the noodles are cooking, prepare the beef by slicing the flank steak into thin strips against the grain. Season with salt and pepper to taste.
In a large skillet or wok, heat 1 tablespoon of vegetable oil over medium-high heat. Add the beef strips and stir-fry until they start to brown, about 3-4 minutes. Remove the beef from the skillet and set aside.
In the same skillet, add another tablespoon of oil. Add minced garlic and ginger, stirring for about 30 seconds until fragrant.
Add the broccoli florets to the skillet, stir-frying for around 3-4 minutes until they are bright green and tender-crisp.
Return the beef to the skillet. In a small bowl, mix together the soy sauce, oyster sauce, cornstarch, and water. Pour this sauce over the beef and broccoli mixture, stirring to coat everything evenly.
Add the cooked lo mein noodles to the skillet. Toss everything together until the noodles are coated in the sauce and heated through, about 2-3 minutes.
Serve immediately, garnished with sesame seeds and sliced green onions if desired.