Very thinly slice the onion with a sharp knife or mandoline. Combine the onion, 2 tsp white wine vinegar and ½ tsp sugar in a bowl. Set aside to pickle, tossing occasionally, until needed. Thinly slice the zucchini lengthwise with a sharp knife or mandoline. Quarter the tomatoes. Crush or finely chop the garlic.
Step 2 — Cook vegetables
Heat a chargrill pan over high heat until hot. Put the zucchini and tomato in a bowl, drizzle with 2 tsp olive oil, season with salt and pepper and toss to coat. Cook the zucchini and tomato, in batches, for 2 mins each side or until tender. Remove the vegetables from the pan and cover to keep warm. Reserve the pan.
Step 3 — Season chicken
Meanwhile, combine the garlic, half the dried oregano* and 2 tsp olive oil in a shallow dish and season with salt and pepper. Put the chicken flat on a board, put your hand on top and halve horizontally into fillets. Add the chicken to the oil mixture and turn to coat.
Step 4 — Cook chicken
Heat the reserved chargrill pan over high heat until hot. Cook the chicken for 2-4 mins each side until golden and cooked through. Remove from the pan and rest for 3 mins.
Step 5 — Toast pita
Reduce the heat to medium-high. Warm 2 pita*, one at a time, for 30 secs each side or until heated through. (Or, heat the pita in the microwave, if you prefer. Put 2 pita* on a plate and microwave for 30 secs or until warmed through.)
Step 6 — Get ready to serve
Slice the chicken. Drain the onion, discarding the pickling liquid. Divide the pita among plates and fill with the zucchini, tomato, chicken and pickled onion. Drizzle with the yoghurt to serve.