Heat the oven to 200C/fan 180C/gas 6. Season the chicken thighs well, put into a deep 30cm x 20cm roasting tin and roast for 40 minutes until golden and crisp.
Meanwhile, melt the butter in a pan and gently fry the garlic, onion and leeks until soft and translucent. Add the chicken stock and wine, and gently simmer for 10 minutes. Add the double cream and lemon juice, and gently simmer for a further 5-10 minutes or until the mixture has thickened.
Take the chicken out of the oven, spoon out any fat, and pour the creamy mixture around the thighs so that the skins remain exposed. Put the lemon slices around the thighs, scatter over the whole rosemary sprigs and put back in the oven for a further 20 minutes. Serve with orzo or rice, if you like.