Slice the beef against the grain into 0.5 cm (1/4 inch) thick slices or 1-cm (1/2 inch) sticks.
Transfer to a small bowl. Add soy sauce, peanut oil, and cornstarch. Gently mix until all slices are coated with a thin layer of sauce.
Marinate for 10 minutes while preparing the other ingredients.
Sauce Preparation
Combine all ingredients for the sauce in a medium-sized bowl. Mix well.
Cooking
Add 1/4 cup water into a large nonstick skillet over medium-high heat until boiling. Add the broccoli and cover. Steam until just tender, about 1 minute. Transfer broccoli to a plate.
Wipe the pan with a paper towel if there’s any water left. Add oil and swirl to coat the bottom. Heat until hot.
Spread the steak in a single layer. Cook without touching for 30 seconds, or until browned. Flip to cook the other side for a few seconds.
Stir and cook until lightly charred and inside is still pink.
Add garlic and ginger. Stir a few times to release flavor.
Return broccoli to the pan. Stir sauce again to dissolve cornstarch completely and pour into the skillet. Cook and stir until sauce thickens, about 1 minute.
Transfer everything to a plate immediately and serve hot.
Slice beef thin against the grain for tenderness. Do not overcook broccoli; steam briefly to retain crispness. Mix sauce well to dissolve cornstarch before use. Use a very hot pan for a good sear on the beef. For a richer color, add a little dark soy sauce. For a lighter sauce, skip the dark soy.