Saute mushrooms and onions in butter until tender.
     Grate in the egg.  Add the cracker crumbs and 
     season to taste.  Mix in enough sour cream to make 
     a thick paste.

     Roll out the pastry on a lightly floured cloth, 1/16 to
     1/8" thick.  Cut into rounds with a 3-4" cutter.  Place
     rounds on greased cookie sheets, put a small ball of
     filling on each (about 3/4" diameter), brush border
     with mixture of egg yolk and 1 Tbsp. cold water, fold 
     over, and seal. Press sealed with tines of fork.  Brush
     top with more of egg yolk mixture, and prick with 
     fork.  Bake in preheated 400* oven until nicely 
     browned, about 20 minutes.  This should make 
     about 3 dozen piroshkis.

     Note 1:  I usually cut out all the dough, count how 
     many rounds I have, and divide the filling into that
     many little balls.  However, extra dough is good 
     rolled out, brushed with egg, pricked, and baked
     as crackers.

     Note 2:  If you plan to freeze them, bake for about 15 
     minutes, until only lightly browned.  Freeze in single 
     layer in plastic bag.  Then reheat on a greased 
     cookie sheet, without defrosting, at 400* for about
     10 minutes.