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1
Preheat the oven to 300.
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2
Halve the lemon.
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3
Cut 3 very thin slices from 1 half; halve the slices.
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4
Squeeze 1 tablespoon of juice from the other lemon half and reserve.
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5
Using your fingers, carefully loosen the skin over the chicken breasts and thighs; do not tear the skin.
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6
Stuff the lemon slices, 8 of the sage leaves and the parsley under the skin.
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7
Season the cavity with salt and pepper, then stuff it with the remaining lemon, 3 sage leaves, 1 thyme sprig and 2 garlic cloves.
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8
Rub 1 tablespoon of the oil over the chicken; season with salt and pepper.
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9
Tie the legs together with kitchen string.
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10
Coat the bottom of a medium roasting pan with the remaining 2 tablespoons of oil.
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11
Set the chicken in the pan, breast side down, and roast for 10 minutes.
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12
Meanwhile, in a small saucepan of boiling water, blanch the Brussels sprouts for 2 minutes.
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13
Drain and let cool slightly, then halve.
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14
Add the Brussels sprouts, beets, turnips, onions, carrots and the remaining sage leaf, thyme sprig and garlic clove to the roasting pan; season with salt and pepper and roast for 1 hour.
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15
Add 3/4 cup of the stock to the pan and roast for 20 minutes longer.
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16
Increase the oven temperature to 450.
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17
Turn the chicken breast side up and baste with the pan juices.
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18
Continue roasting for 10 minutes longer, or until the bird is browned and an instant-read thermometer inserted in the thickest part of the thigh registers 165.
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19
Transfer the chicken to a platter and cover loosely with foil.
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20
Continue roasting the vegetables for another 10 minutes, or until tender.
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21
Transfer the vegetables to the platter and cover loosely with foil.
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22
Set the roasting pan over a burner and bring the pan juices to a boil.
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23
Add the remaining 1/4 cup of stock and the reserved lemon juice and bring to a simmer over moderately high heat, scraping up the browned bits from the bottom of the pan.
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24
Remove the pan from the heat and swirl in the butter.
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25
Season the jus with salt and pepper; transfer to a warmed gravy boat.
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26
Carve the chicken.
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27
Sprinkle the walnuts over the chicken and vegetables and serve with the jus.