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.