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Heat oven to 200C/180C fan/gas 6. Tip the carrots, celeriac, swede, onions and garlic into a large roasting tin with the oil, rosemary leaves and a grinding of black pepper. Toss well and roast for 5-10 mins while you get the chicken ready.
Grate the zest and squeeze the juice from the lemon, set aside and put the lemon shells and the woody stalks from the rosemary inside the chicken. Stir the veg, scatter over the lemon zest and drizzle over the juice, then sit the chicken on top of the veg and roast for 1-1 1/4 hrs until the chicken is tender but still moist. Take the chicken from the oven and leave to rest for 10 mins. Keep the veg in the oven and steam one of the bags of kale (if you're eating this over two days for the Healthy Diet Plan, otherwise steam both).
Squeeze the garlic from the skins, carve the chicken and serve with the vegetables.