In a large frypan, melt butter over low heat. Stir in onions and mushrooms, and saute until onions are soft. Stir in the flour and continue to cook for 4 minutes, stirring constantly.
Pour in stock gradually, and continue to stir over a medium heat until thickened. Stir in asparagus soup, sherry, and chicken. Reduce heat to low, and simmer until the chicken is done and the sauce has thickened, about 10 minutes.
Meanwhile, arrange pastry shells in a large baking tray. Bake in preheated oven until golden brown, about 8 to 10 minutes. Allow to cool slightly before spooning equal amounts of the chicken filling into each shell. Serve immediately.