In 10-inch skillet, melt butter over medium heat. Cook and stir onion in butter just until tender. Add mushrooms; cook 3 minutes, stirring frequently. Stir in egg yolk, salt and pepper. Cool 10 minutes.
Meanwhile, heat oven to 400°F. Unroll 3 pie crusts. With 2 3/4-inch round cutter, cut 10 rounds from each crust.
Spoon rounded teaspoon cooled mushroom mixture onto half of each pie-crust round. Fold dough over filling; press edges with fork to seal. Place on ungreased large cookie sheet. Brush with beaten egg.
Bake 12 to 15 minutes or until light golden brown. Serve warm.