Shred the carrots using a julienne peeler or cut into thin matchsticks. Cut the cucumbers into long batons. Pick the coriander leaves and finely chop the stems. Thinly slice the chilli, discarding the seeds if less heat is desired (the chilli is mild but if you prefer zero heat, omit it or serve at the table).
Step 2 — Pickle carrot
Combine 2½ tbs white vinegar and 2 tsp sugar in a large bowl, season with salt and pepper and stir until the sugar dissolves. Add the carrot and coriander stems, then toss to combine. Set aside to pickle, tossing occasionally, until needed.
Step 3 — Marinate chicken
Combine the pho seasoning and 1 tbs oil in a large bowl. Add the chicken and turn to coat.
Step 4 — Chargrill chicken
Heat a chargrill pan over medium-high heat until hot. Cook the chicken, in batches, for 5-6 mins each side, until golden and cooked through. Remove from the pan. Slice the chicken and put in a clean bowl. Add any resting juices and 2 tsp soy sauce and toss to coat.
Step 5 — Toast buns
Meanwhile, preheat the grill to high. Halve the buns through the middle. Put the buns on a large oven tray, cut-side up, and grill for 1-2 mins until lightly toasted.
Step 6 — Get ready to serve
Drain the carrot, discarding the pickling liquid. Add the cucumber, coriander leaves and chilli to the carrot and toss to combine. Combine the aioli and sweet chilli sauce in a bowl. Spread the buns with sweet chilli mayo and fill with the chicken and pickled vegetables. Serve the burgers at the table.