For the chicken: Preheat the oven to 450 degrees F. Place the chicken breast-side down on a chopping board and, using a good pair of scissors, cut along each side of the backbone to remove it, cutting through the ribs as you go. Then turn it over and flatten the breast down with the heel of your hand. There will be a small crack which means the bird will then be nice and flat. Then put the bird breast-side up in a roasting pan.
Use the tip of a small sharp knife to put enough slits all over the breasts and legs as there are garlic slices and then push the garlic slices into them. Drizzle with the oil, season well with salt and pepper and then pop into the oven for 30 minutes to get some color on the skin.
Meanwhile, mix the mustard, honey, chili powder and thyme leaves together in a small bowl and set aside.
After 30 minutes of cooking, pull the bird out of the oven and reduce the oven temperature to 400 degrees F. To help cool the oven down quickly, you can open the oven door and once the thermostat's red light comes back on shut it. Meanwhile, smear the chicken with the mustard mixture using the back of a spoon. Once the oven temperature is correct, pop the chicken back into the oven until cooked, a further 15 minutes.
To check that the chicken is cooked, insert a sharp knife into the thickest part of the leg and press the knife down a bit. If the juices run clear and are not pink, then the chicken is cooked. Once cooked, lift the chicken out of the pan and place on a warmed platter or board and cover with foil to keep warm. Reserve the pan.
Meanwhile, for the sweet potato and mint mash: Boil the sweet potatoes until the potatoes are just tender but not too soft, about 8 minutes or so (depending on the size of the chunks). A couple of minutes before they are ready, melt the butter in a small frying pan over medium heat and cook the spring onions, stirring occasionally, until softened, about 2 minutes. Drain the cooked potatoes well and then mash well with a masher. Stir in the spring onions and butter and season well with salt and pepper. Pop a lid on to keep it warm.
For the gravy: Holding the roasting pan with an oven cloth, tip it up until all the fat and juices run to one corner. Spoon off as much fat as possible and discard. You should be left with around 3 tablespoons of juices in the bottom of the tin. Pop it on the stovetop over a low heat and stir in the flour until well blended. Add the stock and wine a little at a time, stirring all the time so that it does not go lumpy. Once all the liquid is added, bring the mixture to the boil and cook, stirring regularly, until thickened, 2 to 3 minutes. Pour through a fine sieve into a warmed jug and season to taste with salt and pepper.
For serving: Carve the chicken, arrange the slices on a large serving platter and garnish with fresh thyme sprigs. Spoon the sweet potato mash into a serving bowl and scatter the mint over. Serve at once with the hot gravy.