Add the onions and saute until soft. Add the mushrooms and salt and pepper to taste and cook until soft. Add the wine, tomato puree and stock and continue cooking, stirring, until the liquid is well concentrated; chill.
Beat out the chicken breasts to an even thickness and place each one on a sheet of foil.
Divide the mushrooms mixture between them and dot with the remaining butter.
Fold the foil over the top and seal the edges.
TO FREEZE Place parcels on a baking sheet and freeze, then pack in a plastic bag.
Seal, label and return to the freezer.
TO SERVE Place the foil parcels on a baking sheet. Cook from frozen in a preheated hot oven, 220 C , for about 1 hour.