Heat butter in a large skillet over medium heat. Add leeks; cook and stir until tender but not browned, about 8 minutes. Lower heat to medium-low and add mushroom sage oil and mushrooms. Cook and stir until mushrooms reduce by half, about 15 minutes. Stir in garlic, and let cook until soft but not brown, 2-3 minutes. Deglaze with lemon balsamic, then add the Parmesan cheese, and rosemary. Season to taste with salt and pepper. Stir in crème fraiche. Remove from heat and allow mushroom mixture to cool. It will thicken as it cools.
Preheat oven to 425 degrees. Unfold pastry sheet onto floured surface and lightly roll into a 12 x 15 inch square. Cut into eighteen 2.5 inch squares, you will have a little extra dough. Place a mounded teaspoon of mushroom filling in the center of each square. Fold pastry up to the center from each corner to make a little square parcel. Pinch and slightly twist edges very firmly to seal. Chill in refrigerator until just ready to bake.
Bake 12 to 16 minutes, or until puffed and golden brown. Serve hot or at room temperature.