Cut the rump steak very thinly into 5cm long slices (freezing the steak for 30 minutes beforehand will help with the slicing). Transfer to a bowl and mix well with the soy sauce, sugar, ginger, garlic and black pepper until thoroughly combined. Let it marinate for 30–60 minutes.

In a small bowl, mix the sesame oil, soy sauce and water until combined. Set aside until needed.

Bring a large pan of salted water to the boil. Add the udon noodles to the pan and cook for about 10–12 minutes if using fresh, or follow the packet instructions. Keeping the water in the pan, remove the noodles into a colander and refresh them under fresh cold water to eliminate starchiness and stop them from overcooking. Drain the noodles and keep warm.

Add the broccoli to the same pan and cook for 1minute, then remove from the pan and drain.

Add the sunflower oil to a large wok and heat it through on the highest heat until smoking hot. Add the beef, its marinade and the onion to the wok and cook, stirring constantly, for 1 minute. Add the broccoli and cavolo nero and cook for a further 30 seconds.

Add the cooked noodles to the wok, stirring them together with the other ingredients for a couple minutes until heated through. Finally, stir in the soy sauce and sesame oil mixture, add a generous pinch of chilli (dried red pepper) flakes, mix well and turn off the heat.

Sprinkle with toasted white sesame seeds, if using, and serve immediately.