Preheat the oven to 180°C, gas mark 4. Pat the chicken dry with kitchen paper. Stuff the cavity with the lemon wedges, garlic and thyme. Season well inside and out and rub the butter all over the skin.
Place the chicken in a large roasting tray and roast for 50 minutes. Add the potatoes and Brussels sprouts, drizzle with the olive oil and cook for a further 25 minutes. Scatter over the onion and cook for 15 minutes more, or until the meat is cooked through.
Transfer the chicken to a board and leave to rest. Return the vegetables to the oven for 10 minutes more, until golden brown.
Carve the chicken. Serve with the roasted vegetables.