Wash the beef, pat dry and cut into strips. Mix with 3 tablespoon soy sauce and pepper and marinate covered in the refrigerator for about 1 hour.
Meanwhile, clean the beans and cut into 4 cm long pieces. Bring a saucepan with plenty of water to the boil and blanch the beans in boiling water for about 8 minutes. Drain and chill.
Finely chop the lemongrass, mix with the curry paste, 3 tablespoon soy sauce, fish sauce and sugar.
Heat the oil in the wok and fry the beef briefly, then reduce the heat, add the beans and deglaze with the sauce. Steam for another minute.