Roast chicken with herby mustard sauce

Preheat the oven to 200ºC, gas mark 6. Put the chicken in its bag in a roasting tin, cutting a small cross on top of the bag where indicated. Roast for 1 hour 10 minutes. Remove from the oven and carefully cut the bag along the scissor line shown, then discard this section. Baste the chicken with the cooking juices and season the skin. Roast for a final 20 minutes. The chicken should be cooked through with no pink meat remaining, and the juices run clear. Rest the chicken on a board for 10 minutes.

While the chicken is resting, tip the tin juices into a pan, then simmer to reduce by about half. Turn the heat down to low, add the buttermilk and mustard, then warm through gently, whisking continuously for a few minutes to emulsify. Remove from the heat and stir in the garlic, shallots, cornichons and herbs; season.

Tip the cucumber and radishes into a serving bowl, then toss with a little oil and the cornichon brine; season. Carve the chicken, transfer to a platter and spoon over some of the sauce. Serve with the remaining sauce and the cucumber salad.