Preheat oven to 180C and line a large baking tray with baking paper. Place flatbread on tray, drizzle with 1 tbsp olive oil, scatter with 1 tbsp za’atar and season to taste. Bake for 8-10 minutes until golden brown and crisp. Cool completely, break into shards and store in an airtight container for up to 3 days.
Combine lemon rind, 2½ tbsp oil, half the garlic and remaining za’atar in a large bowl, season to taste. Add lamb, rub well to coat and set aside to marinate for 30 minutes.
Combine lemon juice and remaining garlic in a large bowl, stand for 10 minutes to soften the flavour of the garlic. Add sumac and remaining oil, season to taste.
Preheat a char-grill or barbecue to medium-high heat. Drain cutlets from marinade and grill, turning once, for 4-5 minutes or until browned and cooked medium-rare. Set aside to rest for 5 minutes.
While the lamb rests, add Hass avocado, cucumber, tomato, herbs, spring onion and toasted pita to dressing. Toss to combine and serve with lamb, scattered with crushed pistachios.