About 2 hourse before serving: DOUGH: In large bowl with mixer at medium speed, beat cream cheese, flour and butter or margarine until smooth; shape into two balls; wrap in plastic wrap or aluminum foil and refrigerate 1 hour. FILLING: In 10 inch skillet over medium heat, cook mushrooms and onion in butter or margarine until tender, stirrring occasionally. Stir in sour cream, salt, thyme and flour; set aside. On floured surface with floured rolling pin, roll each half of dough 1/8 inch thick. With floured 2 3/4-inch round cookie cutter, cut out as many circles as possible. Repeat. SCraps of dough may be gathered and rolled out again. Preheat oven to 450 degrees. Onto one half of each dough circle, place a ts of mushroom mixture. Brush edges of circles with some egg; fold dough over filling. With fork, firmly press edges together to seal; prick tops. Place turnovers on ungreased cookie sheet; brush with remaining egg. Bake 12-14 minutes until golden.