Remove stems from 8 mushrooms, set mushrooms aside.
Mince remaining mushrooms and extra stems. In a large skillet over medium heat, melt butter, add mushrooms and onions and cook for 5 minutes or until all liquid is evaporated. Stir in bread crumbs, thyme, salt, pepper and cool.
Prepare puff pastry as package directs. Cut 8 rectangular pieces and reserve scraps. Place 1/3 cup of mushroom mixture in the centre of each pastry rectangle, top with steaks and 1 whole mushroom each.
In a small bowl, beat egg whites and 2 teaspoons of water with a fork. Brush pastry edges. Fold pastry over steak and mushroom; overlap edges and press to seal. Trim extra pastry and roll out scraps, cut into leaf or other designs. Brush backs with egg white and arrange on pastry rectangles.
Place on cookie sheet. Refrigerate for 2-3 hours. Let meat sit at room temperature, about 30 minutes before baking.
Preheat oven to 400°F. In a cup, beat egg yolk with 2 teaspoons of water, brush over pastry. Bake 23 minutes for rare, 25 minutes for medium. Serve immediately.
This dish is delicious served with asparagus tips, baby carrots, and broiled tomato halves.