300 grams beef porterhouse or rump steak, fat trimmed, sliced thinly on the diagonal
1 tablespoon fish sauce
1 stalk fresh lemongrass, white part only, chopped finely
1 clove garlic, crushed
1 tablespoon vegetable oil
1 red shallot, halved, sliced thinly
2 teaspoons white sugar
Place beef, fish sauce, half the lemon grass and half the garlic in a bowl; stir to mix well. Refrigerate, covered, for at least 30 minutes for flavours to develop.
Heat a wok or large heavy-based frying pan over high heat. Add half the oil; stir-fry beef for 4 minutes or until browned. Transfer to a plate. Heat remaining oil in wok; stir-fry shallot, remaining lemon grass and garlic for 2 minutes or until fragrant. Return beef to wok, then add sugar; stir-fry to combine for 30 seconds. Season to taste with pepper.
Combine mayonnaise and chilli sauce in a small bowl.
Split baguette pieces lengthways from the top, without cutting all the way through. Spread sides with mayonnaise mixture; fill baguettes evenly with lemon grass beef, cucumber, carrot, green onion and coriander. Drizzle with extra mayonnaise mixture.
Use one long baguette cut into quarters instead of half baguettes, if preferred.
The lemongrass beef can be made up to 2 days in advance and refrigerated in an airtight container.
Make banh mi the night before or in the morning. Wrap banh mi in baking paper or pack in airtight containers. Transport in cooler bags. Refrigerate until ready to serve.