Preheat the oven to 200C/400F/Gas 6. Tuck the chicken pieces, garlic, thyme and lemons snugly in a roasting tin. Season generously with salt and pepper, then dot over the butter.
Roast for 40 minutes until golden brown and cooked through.
Meanwhile, place the sweet potatoes in a roasting tin and stir in the olive oil, rosemary and some salt and pepper. Roast for 30 minutes until tender and golden.
Roughly mash the sweet potatoes in the tin then stir in the crème fraîche with a wooden spoon.
Divide the chicken between plates, making sure everyone gets some garlic and preserved lemon. Serve the sweet potato mash on the side.