Preheat the oven to 180C/Gas 4.
Bring a small saucepan of water to the boil and add the garlic. Blanch and refresh in ice cold water. Repeat, changing the water.
Heat the oil in a large, lidded casserole dish. Add the vegetables, garlic and herbs and cook for 2-3 minutes. Put the chicken on top and pour in the stock. Put on the lid and roast for 1½ hours, or until cooked through. Remove the lid for the last half hour of cooking.
Meanwhile, for the purée place the cauliflower and milk in a large saucepan and cook for 8-10 minutes. Drain, reserving the milk. Place the cauliflower in a blender and add one ladle full of milk at a time until the purée reaches a smooth consistency.
Bring a large pan of water to the boil and cook the beetroots for 10-12 minutes, or until tender. Drain the beetroots and leave to cool, then peel.
Bring a large pan of salted water to the boil, add the carrots and cook for 2-3 minutes, then drain.
Heat a large frying pan and add the butter, when foaming add the beetroots, carrots and kale. Cook for 2-3 minutes.
Leave the chicken to rest covered in foil. Strain the liquid left in the casserole into a saucepan and reduce over a medium heat until you have the consistency of gravy.
Carve the chicken and place on plates with the vegetables. Pour over the gravy and serve.