Mix the tarragon into the butter or margarine and season to taste with
salt and pepper. Make a slit down the side of each chicken breast and fill
with the flavored butter. Fold the edges together and wrap in the slices
of prosciutto, then chill until the butter is firm.
Arrange the chicken in a roasting tin (pan) and pour the stock over. Cook
in a preheated oven at 200°C/400°F/gas mark 6 for about 30 minutes,
basting occasionally, until tender. Remove the chicken from the tin and
boil the stock until reduced by half. Serve the chicken with the sauce
spooned over and the polenta served separately.