Preheat your oven to 400°.
Line 2 baking sheets with parchment paper. Set aside.
Heat a medium sauté pan over medium high heat, add butter and olive oil, and add the onion and sauté until translucent. Add the mushrooms, herbs, sea salt and pepper and cook until the mushrooms are lightly browned and no liquid remains in the pan.
Stir in the flour and cook for 1 minute, add the wine and cream.
Remove from the heat, and stir in the Gruyere cheese. Chill.
Place a rolling mat on your counter, or a large piece of parchment paper. Add a little flour and lightly roll out the puff pasty, cut into 16 pieces using a pizza cutter or knife.
You should keep the puff pastry as cold as possible, if it starts to stick or becomes doughy, return it to the refrigerator for about 10 min. Keep the second piece of puff pastry in the refrigerator while you work on the first one.
Scoop about 1 t tablespoon of the cold mushroom mixture into the center of the dough (do not overfill or they will leak). Fold the over in a triangle and, using a fork, press the edges together to seal them.
Place the mini turnovers onto your prepared baking sheet, at least 1 inch apart. Place in the refrigerator for 20 minutes and prepare the second batch. Refrigerate the second batch for 20 minutes for best results.
Brush the sealed edges and tops of the turnovers with the egg mixture.
Bake each tray for about 15 minutes, or until the tops are golden brown.
Adapted from Jo-Anna, A Pretty Life in the Suburbs, courtesy of Kendall Jackson