Make the pickled vegetables a day ahead. Combine the fish sauce, vinegar, sugar and 250ml of water in a saucepan and bring to the boil. Remove from the heat and allow to cool to room temperature.
Transfer the mixture to an airtight container and add the papaya, carrot and chillies. Mix well, seal and leave to pickle overnight, or for 24 hours.
The beef can also be marinated well ahead. Combine the fish sauce, palm sugar, annatto oil and kreoung paste in a mixing bowl with a pinch of sea salt. Stir until the salt has dissolved.
Add the beef and mix until well coated. Cover and leave to marinate in the refrigerator for up to 24 hours, but at least 2 hours.
When you’re nearly ready to cook, soak eight bamboo skewers in cold water for 30 minutes.
Heat a barbecue or chargrill pan to medium. Thread the beef onto the skewers and chargrill for 2–3 minutes on each side, or until cooked to your liking.
Slice the baguettes halfway through, creating a pocket. Fill each one with some pickled vegetables and two beef skewers. Pull the skewers out of the rolls and serve.