Marinate the Beef: In a medium bowl, toss the thinly sliced beef with 2 tablespoons of soy sauce and 1 tablespoon of cornflour. Make sure every piece is coated. Set aside to marinate for at least 15 minutes at room temperature.

Prepare the Teriyaki Sauce: In a small bowl or jug, whisk together all the sauce ingredients: 80ml soy sauce, mirin, rice vinegar, brown sugar, minced garlic, grated ginger, and sesame oil. Set this aside. Have your cornflour slurry (1 tsp cornflour mixed with 1 tbsp water) ready nearby.

Sear the Beef: Heat 1 tablespoon of oil in a large wok or frying pan over a high heat until it's shimmering. Add the beef in a single layer (you may need to do this in two batches to avoid overcrowding the pan). Cook for 1-2 minutes per side, just until browned but not cooked through. I find this quick, high-heat sear is crucial for keeping the beef tender. Remove the beef from the pan and set it aside on a plate.

Cook the Vegetables: Add the onion, red pepper, and broccoli florets to the same hot pan. You might need a tiny splash more oil. Stir-fry for 4-5 minutes until the vegetables are tender-crisp – you want them cooked but to still have a bit of a bite. Add the white parts of the spring onions and cook for another minute.

Combine and Thicken: Return the seared beef to the pan with the vegetables. Give the teriyaki sauce a quick re-whisk and pour it all over everything in the pan. Bring it to a simmer.

Finish the Dish: Once the sauce is bubbling, pour in the cornflour slurry while stirring continuously. Let it simmer for another 1-2 minutes, until the sauce has thickened to a lovely glossy consistency that coats the beef and vegetables.

Serve Immediately: Stir through the green parts of the spring onions. Serve the beef teriyaki stir fry immediately over fluffy steamed rice or noodles, garnished with a generous sprinkle of toasted sesame seeds.