Place the steak on a work surface and cover with clingfilm. Use a mallet to beat the steak until it is about 1 cm (½ inch) thick. Cut the beef into thin strips and set aside.
Heat the oil in a large, heavy-based saucepan. Add the beef strips in batches and stir-fry for 2–3 minutes until browned. Remove with a slotted spoon and set aside.
Add the curry paste to the saucepan and stir-fry for 2–3 minutes over a medium heat. Add the coconut milk, stock, fish sauce, lime juice, lime leaves and the beef. Bring to the boil, turn the heat to low and simmer, uncovered, for 1 hour.
Add the courgettes and bamboo shoots and continue to cook gently for 20–25 minutes. Season and remove from the heat. Stir in the basil leaves just before serving with steamed rice.