Put the cucumber, vinegar and sugar in a non-metallic bowl and set aside to pickle lightly. Bring 300ml water to the boil and add 1 tbsp ramen paste and the noodles. If you're using kale, add this to one side of the pan (it will take longer to cook than other greens). Cover and cook for 1 min less than the pack instructions on the noodles.
Mix the remaining 1 tsp of ramen paste with the sliced steak. Add the steak to the noodle mixture, remove from the heat leave to stand for 1 min – the residual heat will cook the thinly sliced steak. If using spinach, pak choi or cabbage, add this to one side of the pan now, cover and leave to stand for another 30 seconds until wilted.
Serve the beef noodles in a deep bowl with as much of the broth as you like, topped with the sesame oil, chilli oil, soy sauce, spring onions, pickled cucumber and any pickling liquid from the bowl.