Take the chicken out of the fridge 30 mins before you want to cook it. Heat oven to 170C/150C fan/gas 3.
Lay the leafy stalks from the cauliflowers in a large roasting tray (to sit the chicken on). Set the cauliflower heads aside.
In a small bowl, mix the olive oil with the garlic, thyme and a little salt and pepper. Carefully insert your fingers under the skin of the chicken breasts and loosen the skin from the breasts. Push most of the olive oil mixture under the skin, spreading it over the breast meat with your fingers. Sit the chicken on the cauliflower stalks.
Drizzle the remaining olive oil mix over the chicken and rub it in well with your hands. Season with pepper and evenly crumble over the stock cube. Tightly cover the tray with foil. Cook for 1 hr 20 mins.
In the meantime, break the cauliflower heads into florets. Once the cooking time is up, take the tray out of the oven and remove the foil. Turn the oven up to 220C/200C fan/gas 7. While it's coming up to temperature, add the cauliflower florets to the roasting tray and pour in the chicken stock.
Put the tray on the bottom shelf of the oven. Roast for 30-40 mins, or until the chicken is golden brown all over and the cauliflower is cooked. Halfway through the cooking time, baste the chicken with the juices and toss the cauliflower.
To make the toasted sourdough, scatter the pieces of sourdough on a small baking tray 20 mins before the end of the chicken cooking time. Drizzle with the olive oil and season. Put on the top shelf of the oven for 15-20 mins, or until golden brown.
Sprinkle the chopped parsley over the sourdough and mix well. Remove the chicken from the oven and leave to rest for 15 mins before carving. Serve the chicken with the roasted cauliflower, cauliflower stalks and toasted sourdough, spooning over the pan juices.