Pour Soy Sauce (2 teaspoon) and Corn Starch (2 teaspoon) over the Sirloin Steak (1.5 pound), then stir till the beef is well coated. Allow to marinate at room temperature for about 15 minutes.
While waiting for the beef to marinade, prepare the sauce by combining Water (1/2 cup), Soy Sauce (1 tablespoon), Oyster Sauce (3 teaspoon), Sesame Oil (1/2 teaspoon) and Brown Sugar (1 teaspoon).
In a separate small bowl, mix the Corn Starch (2 teaspoon) with Water (2 tablespoon). Stir this mixture well to break up the cornstarch clumps.
Heat up Cooking Oil (2 tablespoon) in a wok over medium-high heat. When the wok starts to smoke just slightly, add the beef to the wok in a single layer. Leave the beef untouched for about 15 seconds to allow it to sear before flipping it with your wok spatula. Stir-fry the beef just until it is no longer pink, then remove the beef from the wok.
Add Cooking Oil (1 tablespoon) to the wok, then add the Garlic (2 clove). Stir-fry for just a few seconds, then add the Broccoli Florets (6 cup). Stir-fry for about a minute.
Stir in the sauce, then continue to stir-fry the broccoli in the sauce for another minute or until they reach your desired doneness, the longer you leave the broccoli to simmer in the sauce, the softer they become. Push the broccoli to the sides of the wok, give your cornstarch mixture a quick stir in the bowl, then pour it into the sauce.
As soon as the sauce thickens, return the beef to the wok and give it a final stir before transferring the dish to a serving bowl.
Garnish with Sesame Seeds (1 teaspoon), if using, then enjoy over steamed rice or on its own!