Peel and finely chop the garlic cloves. Add to the yoghurt with 1 teaspoon of salt and mix well. Transfer half of the mixture to a bowl and put in the fridge.
Use the other half of the yoghurt mixture to coat the chicken. Cover it completely with the yoghurt, then put in the fridge for 4 hours.
Peel and finely slice the red onion and put in a small bowl. Add the other teaspoon of salt, the sugar and the vinegar. Add just enough boiling water to ensure the onion is covered with liquid. Leave for 4 hours.
When you are ready to cook, preheat the oven to 180 degrees. Put the chicken breasts on a baking tray and put in the oven for 20 minutes.
Cut the halloumi into 1cm cubes. Add to the baking tray and cook for another 5 minutes.
Use a potato peeler to create courgette ribbons. Add to the baking tray, drizzle with the oil and cook for a final 5 minutes. Put the flatbreads in the oven for the last minute to warm through.
Grate the cucumber and add to the remaining half of yoghurt mixture - stir well.
To compile the flatbreads, spread each bread with a good dollop of the tzatziki. Top with the courgette, sliced chicken breast, halloumi cubes, olives and pickled onions. Eat immediately.