Preheat the oven to 140°C.
Place vegetables in a tray, top with chicken, stuff with lemon and herbs, drizzle with oil, and season.
Roast for 3 hours, basting halfway through.
Increase heat to maximum for 5-10 minutes to crisp the skin.
Remove chicken to rest for 15 minutes, reserving vegetables and juices in the tray.
Strain juices and vegetables through a sieve into a pan, add stock, and bring to a boil.
Melt butter in the roasting tray over low heat and scrape up browned bits.
Whisk in flour to form a paste, then gradually whisk in the hot stock mixture until thickened.
Simmer gravy for 5 minutes, season, and strain into a jug.
Carve the chicken and serve with gravy and roasted vegetables.