In large bowl, with electric mixer at medium speed, beat cream cheese, ½ cup butter and 1 ½ cups flour until soft dough forms. Wrap dough in waxed paper, chill at least 1 hour.
In medium skillet, in 3 tablespoons butter, saute mushrooms and onion until tender. Blend in salt, thyme, and 2 tablespoons flour, stir in sour cream, chill.
On floured surface, roll half of dough in 15 inch circle (about 1/16" thick), cut into twenty 2 ¾" circles. Roll scraps into ball, chill.
On one half of each circle, place teaspoonful of mushroom mixture. Brush edges with egg, fold other half over filling, with fork, press edges together, prick tops to let out steam, place on ungreased cookie sheet. Repeat with rest of dough, scraps, filling. Brush with egg, cover, chill.
Preheat oven to 450 deg F. Uncover turnovers, bake 12 minutes or until golden. Makes about 50.