Heat the oven to 190°C (375°F) Gas 5.

Pat the chicken dry with absorbent paper, then sprinkle inside and out with salt and pepper.

Place half the margarine inside the chicken with half the tarragon and the onion halves. Truss with thread or fine string.

Place the chicken in an ovenproof dish and spread with the remaining margarine. Sprinkle over the remaining tarragon, then pour the vermouth around the chicken.

Roast in the oven for 1 1/4 hours or until the chicken is tender and the juices run clear when the thickest part of the thigh is pierced with a skewer. Baste occasionally during the cooking time.

Remove the chicken from the dish and discard the trussing thread. Place the chicken on a warmed serving platter and keep hot in the lowest possible oven.

To make the sauce: pour the juices from the chicken into a saucepan. Stir in a pinch of paprika and the tomato puree. In a bowl, blend the cornflour to a paste with the milk, gradually stir in the chicken stock and add to the pan. Stir well to combine, bring to the boil, then simmer for 2 minutes, stirring constantly. Taste and adjust seasoning, then add lemon juice to taste, for a sharper flavour.

Pour a little of the sauce over the chicken and sprinkle with the remaining paprika. Serve at once, with the remaining sauce handed separately.