Beat cream cheese and 1/2 cup butter at medium speed with an electric mixer until creamy; gradually add flour, beating well.
Divide dough in half, and shape each portion into a ball; cover and chill 1 hour.
Melt 3 tablespoons butter in a large skillet. Add mushrooms and onion; sauté until tender. Stir in sour cream and next 3 ingredients; set aside.
Roll 1 dough portion to 1/8-inch thickness on a lightly floured surface; cut with a 2 1/2-inch round cutter, and place on greased baking sheets. Repeat procedure with remaining dough.
Spoon 1 teaspoon mushroom mixture onto half of each dough circle. Moisten edges with beaten egg, and fold dough over filling. Press edges with a fork to seal; prick tops. Brush turnovers with beaten egg.