Pour the beef stock into a large pan with 500ml cold water. Add half the chilli and the Thai paste and bring to a simmer. Add the green beans, peas and noodles and simmer gently for 5 minutes.
Meanwhile, fry the steak for 2 minutes on each side in a large frying pan, or longer if you prefer it with no pink in the middle. 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.