Cut the beef into strips. Heat the oil in a wok and stir-fry the beef in batches for approx. 1 min. per batch. Remove and season. Slice the lettuce into strips, stir-fry for approx. 2 mins., remove.
Halve the lemon grass and crush it. Deseed the chilli and slice it into rings. Pour the bouillon and coconut milk into the wok, add the lemongrass and chilli, and bring to the boil. Add the noodles and cook until al dente. Add the meat and vegetables and heat through. Drizzle with lime juice.