Slice the steak into 1cm strips. Cover and set aside.
Place the coriander, spring onions, lemongrass, garlic cloves, ginger and chilli in the bowl of a food processor and pulse until you have a coarse paste.
Place the noodles in a large bowl and add just enough boiled water to cover. Leave to soak for 5 mins.
Meanwhile, heat the sunflower oil in a wok or large frying pan and add the strips of beef and quickly stir-fry for a few mins until browned. Add the paste and green beans and fry for about 2-3 mins, tossing together well, until all the beef strips are coated in the paste.
Add the soy sauce, coconut milk and cabbage and cook for a further 2-3 mins or until the beef is cooked through.
Drain the noodles and divide between two serving bowls. Spoon over the beef and sauce and squeeze over some lime juice to taste.