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1
Heat the oil in a wok over high heat until shimmering. Add the Southern prik khing paste and fry, pressing and folding it continuously so the paste cooks from the inside out rather than scorching only on its surface, until it deepens to a dark red-orange and the oil visibly separates and pools at the edges. The moment that oil breaks free and the kitchen fills with a deep, roasted-chilli fragrance, the paste is ready.
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2
Add the thin-sliced or minced beef in a single addition, tossing and pressing vigorously so every surface is coated in paste at once, which prevents the meat from steaming in its own juices before the paste can grip it. Cook, continuing to stir and fold over high heat, until the beef is cooked through and shows no remaining pink.
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3
Add the long beans and continue to stir-fry over high heat, folding them through the beef and paste so they pick up colour and flavour rather than simply softening in their own steam. Cook until the beans are tender but retain a slight bite — they yield to pressure without snapping. Pull them too soon and the raw edge of the bean fights the deep paste; hold them too long and the texture that carries the dish is gone.
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4
Continue cooking, reducing the heat slightly if necessary, until all liquid has driven off and the paste forms a dry, caramelised coating that clings to the beef and beans with no pooling liquid visible in the wok, so each piece is lacquered rather than stewed. Scrape and fold constantly at this stage, as the sugars in the paste will catch and turn bitter the moment your attention lapses.
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5
Remove from heat, add the kaffir lime leaves, and toss once to distribute them through the hot mixture so the residual heat coaxes out their aromatic oils without cooking away their brightness. Serve immediately.