Mix all the ingredients for the marinade in a bowl. Add beef to the marinade. Stir to coat and leave for 20 minutes.
For the stir fry:
Peel outer lemongrass layers away to reveal the white core and cut into 2 pieces. Crush with a knife to release the oils. Cut peppers into 2.5cm (1 in) pieces.
Heat wok or large frying pan over high heat until hot. Add the groundnut oil when the wok is very hot. Remove beef from marinade and add to the pan. Stir fry for 2 minutes, until browned. Remove the meat and leave to drain.
Leave 1 tablespoon of oil in the wok, reheat and add lemongrass, ginger, shallots and garlic. Stir fry for 20 seconds. Add peppers, soy sauce, rice wine, sugar and salt and ground pepper. Stir fry for 3 minutes. Return meat to the wok and stir fry for another 4 minutes.
Remove the lemongrass pieces and discard. Drizzle with sesame oil and serve.
This recipe calls for lean beef steak. A cut such as rump or fillet would well in this recipe.