Pour the beef stock into a large pan along with 500ml cold water. Add half the chilli and the Thai paste, and bring to a simmer. Add the green vegetables and noodles and simmer gently for 5 minutes.
Meanwhile, in a large frying pan, fry the steak for 2 minutes on each side, or longer if you prefer it with no pink in the middle, then set aside to rest for 5 minutes. Cut into thin slices, then add to the soup with the beansprouts to heat through.
Ladle into soup bowls and garnish with the remaining chilli and the coriander.