Slice 1 pound of flank steak against the grain into thin strips (about 1/4 inch thick). Pat the slices dry with paper towels and season lightly with a dash of salt and pepper. In a medium bowl, whisk together 1/4 cup water, 1 teaspoon soy sauce, 1 tablespoon corn starch, and 1/2 teaspoon baking soda until well combined. Add the sliced beef and toss to coat evenly. Let marinate for at least 15 minutes while you prepare the other ingredients.
In a small bowl, whisk together 1/2 cup beef stock, 2 tablespoons soy sauce, 2 tablespoons honey, 1 tablespoon oyster sauce, 1 tablespoon sesame oil, 1 tablespoon corn starch, 2 teaspoons rice vinegar, 3 minced garlic cloves, 1 teaspoon sage, 1/2 teaspoon salt, and a dash of pepper until well combined and the cornstarch is fully dissolved.
Bring water to a boil in your wok or a large pot. Add 3 cups of broccoli florets and boil for 3 minutes until bright green and slightly tender. Drain in a colander and rinse with cold water to stop the cooking process. Set aside.
Heat 1 tablespoon of light olive oil in your wok over medium-high heat until shimmering. Remove the beef from the marinade, letting excess drip off. Add the marinated beef in a single layer (work in batches if necessary to avoid overcrowding). Cook for 1-2 minutes per side until nicely browned but not fully cooked through. Remove the beef from the wok and set aside.
Add another 1/2 tablespoon of light olive oil to the wok. Add 1/2 sliced onion and sauté for 2-3 minutes until they begin to soften and become translucent.
Pour the sauce mixture into the wok with the onions. Bring to a simmer and cook for 1-2 minutes, stirring frequently, until the sauce begins to thicken. Return the beef to the wok and stir to coat with the sauce. Cook for about 1 minute. Add the blanched broccoli and toss everything together until well coated with the sauce and heated through, about 1-2 minutes more. Transfer to a serving dish, sprinkle with 1 tablespoon sesame seeds, and serve immediately over white or brown rice.