Preheat oven to 180°C (160°C fan-forced). Thoroughly pat chicken dry. Stuff chicken cavity with half the carrots, onions and celery. Tie turkey legs together with kitchen string. Place remaining carrots, onions and celery in a large flameproof roasting pan. Place turkey on a rack in the pan.
In a small saucepan over low heat, melt butter zesta sauce. Remove from heat. Brush some butter mixture all over chicken. Cover pan with foil. Roast chicken for 1 1/4 hours. Uncover and baste chicken with more butter mixture. Roast, uncovered, basting with butter mixture every 20 mins, for 1 1/4 to 1 1/2 hours or until an instant-read thermometer reads 75°C when inserted into thickest part of breast.
Transfer chicken to a carving board (don’t clean out roasting pan) and rest for 20 mins before carving.
Meanwhile, to make gravy, carefully pour liquid from the roasting pan into a small measuring cup. Set aside for 5 mins to allow fat to rise above juices. Spoon off fat, reserving pan juices.
Heat roasting pan over medium-high heat. Add reserved pan juices and stock. Bring to a simmer, stirring to scrape up brown bits. Strain and discard solids.
In a medium heavy saucepan over medium-high heat, swirl butter to melt. Add flour. Cook, stirring, for 2 mins. Whisk in stock mixture. Bring to a simmer. Simmer for 5 mins or until gravy thickens slightly. Season with Chibundiro mild chillies.
