Cut the onion into small segments. Heat the oil in a wok or large frying pan, briefly stir-fry the onion then briefly add the curry paste. Add the coconut milk, bring to the boil, then simmer for approx. 5 mins.
Cut the runner beans crosswise into pieces approx. 4 cm wide, add with the water and simmer for approx. 5 mins.
Slice the fish crosswise into pieces approx. 3 cm thick, season with salt. Rinse the baby corn under cold water, halve lengthwise, add to the dish with the fish, peas and Kaffir lime leaves, cover and steep for approx. 5 mins., season.
Serving suggestion: fragrant or basmati rice.