Preheat the oven to 200C/180C Fan/Gas 6. Peel and finely grate the garlic. Mix the garlic, oil and mixed herbs in a large bowl. Set aside a little of the herby oil for the chicken. Cut the sweet potatoes into thick chips then peel the onions and cut into thin wedges. Combine the sweet potatoes and onions in the large bowl with the herby oil, toss to coat then tip into a large roasting tray.
Slather the chicken in the remaining herby oil and place on top of sweet potatoes and onions. Roast in the oven for 40 minutes or until cooked through. Give the tray a shake half way through to make sure the sweet potato doesn’t stick to the roasting tray. The chicken is cooked if the juices run clear when a skewer is inserted into the thickest part of the thigh.
Remove the tray from the oven. Leave it to rest for 10 minutes for the chicken to become tender.
Meanwhile, mix together the yoghurt and hot sauce. Serve the chicken and vegetables with the yoghurt on top.