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Heat oven to 200C/180C fan/gas 6. Spread the onion and half the garlic cloves around the base of a large roasting tin. Pick the tops off all the rosemary sprigs and set them aside, then stuff the remaining stalks into the chicken cavity and sit it on top of the onions. Drizzle with 2 tbsp of the oil and season well. Cover with foil and roast in the oven for 1 hr 20 mins, removing the foil for the last 20 mins.
Just after the chicken goes in, put the vegetables in a second roasting tin along with the remaining garlic and the rosemary tops. Drizzle with the remaining oil, season and put in the oven underneath the chicken for 1 hr-1 hr 10 mins or until soft and lightly browned.
Check the chicken is cooked by piercing the thickest part of the thigh with a skewer – the juices should run clear; if not, put it back in the oven and test again after another 10 mins. Carefully remove the chicken from the roasting tin and rest it on a board 15-20 mins.
As the chicken rests, keep the vegetables warm and make the gravy. Put the roasting tin with the onions on a hob over a low heat, add the flour, mix well until it forms a paste, then pour in the stock, stirring constantly until thickened. Simmer for 5 mins, then season to taste. Strain into a gravy boat or heatproof jug and serve with the chicken, vegetables, cabbage and roast potatoes.