First, make the stuffing.
Melt ½ oz (10 g) of the butter in a small saucepan and cook the finely chopped onion for about 5 minutes or until softened, then add the remaining butter and allow it to melt. Now transfer the onion and the buttery juices to a bowl, add the breadcrumbs, then stir in the crushed garlic, chopped parsley, tarragon and grapes. Taste and season with salt and pepper. Next, loosen the breast skin of the chicken a little to make a pocket for the stuffing. Place the stuffing in the pocket but don’t overfill – if you do, the skin will burst when cooking. Secure the skin flap underneath with a small skewer.
Season the chicken all over with lots of salt and pepper, then rub it all over with the 2 oz (50 g) butter, and cover the breast with buttered foil. Place the chicken in a roasting tin and cook for approximately 1 hour, 20 minutes, basting the meat with the buttery juices every 20 minutes or so. Remove the foil half an hour before the end of the cooking time to brown the breast. To test if the chicken is cooked, pierce the thickest part of the thigh with a thin skewer: if cooked, the juices will run golden and clear.
Drain the chicken thoroughly and keep warm while you make the gravy. Carefully tilt the roasting tin and pour off some of the excess fat – you need to leave about 1-1½ tablespoons of fat behind. Then put the tin over direct low heat turned fairly low when the juices begin to sizzle, and blend in the flour. Keep stirring and allow the flour to brown before gradually adding the wine and the stock to make a thin gravy.
Taste and season with salt and pepper. Carve the chicken and serve, passing the gravy round separately.
Pre-heat the oven to gas mark 5, 375°F (190°C).