Heat the oven to 180ºC. Toss the zucchini, onion and garlic with the olive oil, and tip into a large, shallow baking dish. Season well and roast for ten minutes.
Stir in the cherry tomatoes and sit the halloumi slices on top. Sprinkle over Italian herbs and chilli flakes, if using. Cook for another 15 minutes until the tomatoes start to burst and release juice, and the halloumi is turning golden.
Squeeze the garlic cloves out of their skins and mash into the oil and juices in the baking dish, stirring it in. Spoon everything onto plates and serve with a sprinkle of fresh herbs.