Remove the chicken giblets. Rinse the chicken inside and out. Pat the outside dry. Place the chicken in a roasting pan. Liberally salt and pepper the inside of the chicken. Stuff the cavity with thyme, both halves of the lemon, and all the garlic. Brush the outside of the chicken with the butter and sprinkle again with salt and pepper. Scatter the onion slices around the chicken. Then follow with potatoes and carrots.
Roast the chicken for 1½- 2 hours or until the juices run clear when you cut between a leg and thigh. Remove to a platter and cover with foil while preparing the gravy.
Remove all the fat from the bottom of the pan, reserving 2 tablespoons in a small cup. Add the chicken stock to the pan and cook on high heat for 5 minutes, until reduced, scraping the bottom of the pan. Combine the 2 tablespoons of fat with the flour and add to the pan. Boil for a few minutes to cook the flour. Strain the gravy into a small saucepan and season it to taste. Keep it warm over a very low flame while you carve the chicken.
Slice the chicken onto a platter and serve immediately with the warm gravy.
Recipe by The Gingham Apron at https://www.theginghamapron.com/roasted-chicken-vegetables/