1. To make the toum,place the garlic cloves,egg white,lemon juice and a pinch of salt in the bowl of a small food processor. Blend until the garlic is finely minced. With the machine running,slowly pour in the oil until it forms a thick,mayonnaise-type paste. Scoop it out into a bowl and set aside.
2. Season the chicken thighs liberally with salt and set aside. Combine the marinade ingredients in a bowl and mix to combine. (Reserve the excess toum for later.) Add the chicken to the marinade and toss to combine. Allow to sit at room temperature for at least 30 minutes.
3. Preheat a charcoal grill or grill pan. Grill the chicken for 3-4 minutes per side until nicely charred and fully cooked through. Set it aside to rest while you prepare the salad.
4. To make the salad,cut the tomatoes and cucumbers into large wedges. Leave the chillies whole (if using). Thinly slice the red onion and rinse the onion slices under cold running water for 5 seconds. Combine tomatoes,cucumbers,chillies (if using) and red onion in a bowl and toss with the lemon juice,olive oil,parsley leaves,salt and black pepper.
5. Mix the yoghurt with 2 tablespoons of the remaining toum (more if you like it really garlicky!) and serve alongside the chicken. Warm your pita bread on the hot grill and serve everything together.
Note: Leftover toum can be used alongside theseTurkish-style sausage flatbreads