01 - In a small mixing bowl, whisk together the low-sodium soy sauce, oyster sauce, cornstarch, brown sugar, water, sesame oil, and crushed red pepper flakes (if using) until well combined. Set this sauce mixture aside.
02 - Heat 1 tablespoon of the vegetable oil in a large skillet or wok over medium-high heat. Add the thinly sliced flank steak and stir-fry for 2 to 3 minutes, until it is browned on the exterior but not fully cooked through. Remove the seared beef from the skillet and set it aside.
03 - Add the remaining 1 tablespoon of vegetable oil to the hot skillet. Introduce the broccoli florets and stir-fry for 2 to 3 minutes, until they become bright green and are slightly tender-crisp.
04 - Incorporate the minced garlic and grated fresh ginger into the skillet with the broccoli. Continue to stir-fry for an additional 30 seconds, or until the aromatics become fragrant.
05 - Return the seared beef to the skillet with the broccoli and aromatics. Pour in the reserved sauce mixture, stirring continuously to ensure all ingredients are evenly coated.
06 - Continue to cook the stir-fry for 2 to 3 minutes, allowing the sauce to thicken to a glossy consistency and the beef to cook through completely.
07 - Remove the beef and broccoli stir-fry from the heat. Garnish generously with the sliced green onions and toasted sesame seeds (if desired) before serving immediately.