1. Preheat the oven to 220˚C/200˚C fan/gas mark 7. With some kitchen towel, pat the outside the chicken dry.
2.Liberally salt and pepper the inside of the chicken. Put the lemon, garlic and half of the thyme along with a pinch of salt and pepper into the cavity of the chicken. Set aside the rest of the thyme.
3.Spread half of the butter over the outside of the chicken and season with salt and pepper. Place the onion and carrots in a large roasting pan. Toss with salt, pepper, thyme leaves and olive oil. Spread around the bottom of the roasting pan and place the chicken on top.
4.Roast for 1½ hours, or until the juices run clear when you cut between a leg and thigh. Remove the chicken and vegetables to a platter and cover with foil for about 20 minutes.
5.To make the gravy – heat the roasting tin over medium-high heat. Mix together the flour and remaining butter before whisking it into the pan juices in the roasting tin. Bring to a simmer until thickened.
Carve the chicken and serve with the gravy, roasted vegetables and roast potatoes.