Melt the butter in a large skillet over high heat, add the beef, onions and garlic, and saute for 5 minutes, until the onions are golden brown.
Add the wine to deglaze the pan. Add the mushrooms, basil, parsely, thyme, salt, and pepper, and cook until the liquid had evaporated. Stir in the brown sauce and cook for about 5 minutes more, until all liquid has evaporated again.
Remove from the heat. Spread the mixture onto a baking sheet and let it cool for 10 minutes. Refrigerate for about 1 hour to cool completely.
Preheat the oven to 450ºF.
On a lightly-floured surface, roll out the Puff Pastry to 1/8 inch thickness. Cut out eight 10-inch squares and brush the edges with the egg mixture.
Place 1 cup of the meat mixture in the center of each square. Fold each square over diagonally to form a triangle and press the edges firmly to seal. Using a fork, gently prick the center of each triangle to allow steam to escape. Lightly brush the edges again with the egg mixture.
Arrange the triangles 2 inches apart on a greased baking sheet(s).
Bake for 8 to 10 minutes, until golden brown.
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