Whisk together soy sauce, mirin, brown sugar, and rice vinegar. Set aside.
Cook The Beef
Heat the oil in a large wok or pan over medium-high heat. Add garlic and ginger and cook for 30 seconds until fragrant. Add the beef and cook for 6–8 minutes, breaking it up and letting it brown and caramelise.
Glaze It
Pour in the sauce and toss to coat. Simmer for 3–4 minutes, until the mixture is thick, sticky, and glossy. Stir in the butter to finish and turn off the heat. Garnish with sesame seeds and sliced spring onion.
Cook The Noodles
Cook noodles for 2 minutes less than the packet instructions, then drain. In a pan, heat the sesame oil and garlic for 30 seconds. Add the noodles and soy sauce, and toss for 1 minute, until glossy and fragrant.
Blanch The Greens
Cook broccolini in boiling salted water for 2 minutes. Drain and toss with sesame oil, salt and pepper.
Assemble
Add noodles to each container, top with greens, then finish with sticky beef and a spoonful of glaze. Sprinkle with sesame seeds and spring onion.