Put the chicken flat on a board, put your hand on top and halve horizontally into fillets. Lightly score each side with a sharp knife, taking care not to cut all the way through. Put the chicken in a dish and spread all over with the chipotle (reduce for less of a chilli kick). Marinate for at least 5 mins.
Step 2 — Prep vegetables
Meanwhile, using a clean board, very thinly slice the onion with a sharp knife or mandoline. Peel the carrot and zucchini into ribbons with a vegetable peeler. Pick the parsley leaves, discarding the stems.
Step 3 — Make pickles
Combine 1 tbs white wine vinegar, 2 tsp sugar and ½ tsp salt in a large bowl. Add the onion, carrot and zucchini and toss to combine. Set aside to pickle, tossing occasionally, until needed.
Step 4 — Cook chicken
Heat 2 tsp olive oil in a medium frypan over medium-high heat. Cook the chicken, turning regularly, for 4-8 mins until golden and cooked through (if the chicken is browning too quickly, reduce the heat to medium). Remove from the pan and rest for 3 mins.
Step 5 — Toast buns
While the chicken is resting, preheat the grill to high. Halve the buns through the middle. Put on an oven tray, cut-side up, and grill for 2-3 mins until lightly toasted.
Step 6 — Get ready to serve
Drain the pickles, discarding any liquid. Add the parsley and toss to combine. Thickly slice the chicken. Spread the buns with the aioli. Divide the chicken and some of the pickles among the bases, then sandwich with the bun tops. Serve with the remaining pickles on the side.