This is my attempt at making a dish I've eaten and loved at Krua Apsorn, a restaurant I visit almost every time I'm in Bangkok.
The chillies for this dish are sold at shops specialising in Thai ingredients. They are smaller than banana chillies, are yellow to orange in colour and the flavour is usually very mild (although you will occasionally get a batch that's very spicy).
Kaffir lime leaves are double leaves - they are attached to each other as a pair. You need three to four pairs for this dish.
If using frozen crab meat, let it thaw in the fridge. Cut the long beans into 1cm (7/16in) pieces. Cut the chillies in half lengthwise, then scrape out and discard the seeds. Chop the chillies and the garlic, then pound them in a pestle until they are roughly crushed. In a small bowl, stir together the fish sauce, sugar and white pepper.
Heat a wok over a high flame and when it’s hot, add the cooking oil. Put the kaffir lime leaves in the wok and stir-fry until they darken slightly (about 15 seconds), then put them in a bowl. Heat the wok again, add the long beans and stir-fry for about a minute, or until they are crisp-tender. Put the long beans in the bowl with the kaffir lime leaves.
Heat the wok again over a high flame and add a little more oil, if needed. When the wok is hot, add the chilli and garlic mixture and stir-fry for about 30 seconds, or until fragrant. Mix in the crab meat and fish sauce/sugar/pepper mixture and stir-fry for a few minutes, or until the crab is almost cooked. Add the long beans and kaffir lime leaves and stir-fry until the crab is fully cooked. Taste for seasonings and adjust, if necessary. Scoop the mixture into a dish and serve immediately.