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Heat oven to 160C/140C fan/gas 3. Put the potatoes in a large roasting tin with the onion. Pour over 2 tbsp olive oil and roast in the oven for about 30 mins.
Add the chickpeas, pepper, romanesco, tomatoes and garlic. Drizzle with 2 tbsp oil, then roast for a further 20-25 mins until everything is cooked and browning nicely. Toss together briefly and put the halloumi slices on top. Put it under the grill for 5-10 mins, or until the cheese is melting and browning (keep an eye on it). Scatter over the basil leaves to serve.