Heat the oil in a large pan, then fry the onion and red peppers until soft. Stir through the curry paste, then cook for 2 minutes. Add the mushrooms, coconut milk, beef stock, lime juice and fish sauce. Simmer for 15 minutes.
Meanwhile heat a griddle pan. Oil and season the sirloin steaks, then cook for 2 minutes on each side (longer for well done). Rest for 5 minutes, then slice. Ladle the soup into bowls, top with the steak, spring onions, red chilli and fresh coriander. Serve with lime wedges.