Preheat the oven to 200oC, gas mark 6. Place the chicken in a large roasting tin.
Mix 1 tbsp wholegrain mustard with 2 tsp Dijon and half the chives. Loosen the skin over the chicken breast and spoon the mustard mixture under the skin smoothing the surface out. Roast for 1 hour.
Cook the parsnips in boiling water for 5 minutes, drain and toss in 1 tbsp oil and the Parmigiano Reggiano.
Cook the parsnips in boiling water for 5 minutes, drain and toss in 1 tbsp oil and the Parmigiano Reggiano.
Mix the remaining wholegrain mustard and 1 tsp Dijon and brush over the whole chicken 10 minutes before the end of cooking.
Meanwhile, heat 1 tbsp oil in a large frying pan and fry the leeks, covered for 5-6 minutes.
Stir in the crème fraîche and remaining chives then season. Check the chicken is cooked through (with no pink meat) and serve with the parsnips.