Slice the beef thinly against the grain. Cut on a bias for larger slices.
Add the BEEF MARINADE ingredients to the beef slices and mix well until all the liquid is absorbed. Marinate for at least 30 minutes.
In a small bowl, combine cornstarch and water to make the cornstarch slurry. Set aside.
Bring a medium sized pot of water to a boil. Add the broccoli florets and blanche quickly for about 15-20 seconds. Drain and set aside.
Heat a wok or skillet over high heat. Add a tablespoon of oil and swirl it around to coat the pan.
Arrange the beef in a single layer and allow it to brown on one side. Flip the beef over and brown the other side. Add garlic and quickly stir-fry.
Drizzle in light soy sauce, dark soy sauce and oyster sauce. Toss well to coat the beef and allow the sauce to thicken and caramelize.
Before the sauce begins to burn, carefully pour in water and allow it to come to a boil.
To thicken the sauce, push the beef and broccoli to one side to separate from the sauce. Give the cornstarch slurry a good stir and drizzle it into the sauce. Quickly stir vigorously to avoid clumping. Add cornstarch slurry a teaspoon at a time until you reach your desired consistency.
Give all the ingredients a final toss to combine. Serve Panda Express Beef and Broccoli copycat with steamed rice.