Preheat oven to 200° C.
Brush chicken with ½ tbs olive oil and season with pepper. Thread the chicken onto a rotisserie or roast on a rack in a roasting pan in the oven for 20 minutes. Reduce heat to 180°C and roast for another 50 minutes.
Toss the remaining oil and nigella seeds over the carrots and place on a separate roasting pan. Place in oven for the last 15 minutes of cooking the chicken.
Transfer chicken to a plate to rest and cover loosely with aluminium foil.
Increase oven temperature to 200°C and cook the carrots for another 10 minutes. Transfer carrots to a bowl. Drain the fat from the roasting pan into a jar and discard. To make gravy, whisk the cornflour into the remaining juices in the roasting pan on the cooktop over medium heat. Cook for a minute or so, then slowly whisk in the chicken stock
Meanwhile, steam the brussels sprouts for approximately 7 minutes or until tender. (If the gravy becomes too thick, add a little of the sprout juice.) Season the gravy with salt and pepper.
Serve the chicken with gravy, carrots, and sprouts.