Preheat the oven to 220ºC, gas mark 7 and line a large baking tray with parchment. Bring a large pan of water to the boil, add the potatoes and cook for 10 minutes. Meanwhile, combine the halloumi, aubergine and courgette in a bowl. Add the cumin, zaatar and olive oil, season and mix well to coat.
Drain the potatoes and set aside briefly to steam dry. On a board, press each potato down with a fork, until slightly crushed and the skin splits.
Put the potatoes onto the baking tray with the halloumi and vegetables, and shake it so the potatoes take on a little of the oil. Roast in the oven for 20 minutes, or until the potatoes are crisp. Meanwhile, combine the yogurt, garlic and mint in a small bowl and season. Serve the traybake scattered with mint leaves, the salad onions and the garlicky yogurt for dolloping.
