Preheat the oven to gas 6, 200°c, fan 180°c. Put the leeks and garlic in the bottom of a shallow, kidded casserole dish. Mix together the white wine, stock, thyme, cream and mustard. Pour over the leeks and stir to coat. Score each chicken leg 3 times with a sharp knife and place on top of the leeks. Season, cover with the lid and cook in the oven for 45 minutes.
Turn the oven down to gas 4, 180°c, fan 160°c. Remove the lid and cook for a further 30 mins until the chicken is golden and the juices run clear.
Meanwhile, blitz all of the salsa ingredients in a food processor with 1tbsp water until a rough paste forms.
Serve the chicken legs with the leeks and the salsa verde drizzled over. Serve with mashed potato, if you like.