Prepare the beef: Slice the beef sirloin or flank steak very thinly against the grain. Aim for pieces about 0.5cm (1/4 inch) thick. This is crucial for tender beef. Place the sliced beef in a medium bowl.

Marinate the beef: Add 30ml (2 tbsp) of soy sauce, 10g (1.5 tbsp) of cornstarch, and 5ml (1 tsp) of sesame oil to the bowl with the beef. Mix thoroughly to ensure all beef strips are coated. Let it marinate for at least 15 minutes at room temperature, or up to 30 minutes.

Prepare the broccoli: Wash the broccoli and cut it into bite-sized florets. If the stems are thick, you can peel them and slice them thinly. Set aside.

Prepare the sauce: In a small bowl, whisk together the remaining 30ml (2 tbsp) soy sauce, 10g (1 tbsp) cornstarch, 30ml (2 tbsp) oyster sauce, 15g (1 tbsp) brown sugar, 60ml (4 tbsp) water, and the remaining 10ml (2 tsp) sesame oil. Stir until the cornstarch and sugar are dissolved. Set aside.

Cook the beef: Heat 15ml (1 tbsp) of vegetable oil in a large skillet or wok over medium-high heat until shimmering. Add the marinated beef in a single layer (cook in batches if necessary to avoid overcrowding). Stir-fry for 1-2 minutes per side, until browned but not fully cooked through. Remove the beef from the skillet and set aside.

Stir-fry the aromatics: Add the remaining 15ml (1 tbsp) of vegetable oil to the same skillet over medium-high heat. Add the minced garlic and grated ginger. Stir-fry for about 30 seconds until fragrant, being careful not to burn the garlic.

Cook the broccoli: Add the broccoli florets to the skillet. Stir-fry for 2-3 minutes until they start to turn bright green and slightly tender-crisp. Add about 30ml (2 tbsp) of water to the skillet, cover, and steam for 1-2 minutes, or until the broccoli is tender-crisp to your liking. Be careful not to overcook it, as it should remain slightly firm.

Combine and thicken: Return the cooked beef to the skillet with the broccoli. Give the prepared sauce a quick whisk and pour it over the beef and broccoli. Stir continuously as the sauce heats and thickens, coating everything evenly. This should take about 1-2 minutes.

Serve: Remove from heat immediately once the sauce has thickened and everything is well combined and heated through. Serve the beef and broccoli hot over cooked white rice.