Start by coating the beef evenly in cornflour. Mixing well in a large bowl.
Add half of the rapeseed oil into the pan and increase the heat to high. Evenly distribute half of the beef across the pan and fry for 2 minutes on each side.
Repeat with the second batch, setting the beef aside. Season with salt.
Next, add the remaining rapeseed oil into the pan, along with the mange tout and onion. Season with salt and chilli flakes and fry for a couple of minutes.
To that, layer the udon noodles into the pan, then the pak choi. Add a splash of water into the pan and cover with a lid, reducing the heat to medium. Leave to cook for 4 minutes, or until the noodles are loose enough to separate without breaking.
Remove the lid and add the beef, soy sauce, sweet soy sauce, rice vinegar and an extra pinch of salt. Stir to incorporate all of the flavours. Serve with a sprinkle of optional coriander and a serving of broccoli.