Preheat oven to 450. Brush mushrooms well with a damp cloth to remove dirt or grit, do not soak in water. Slice mushrooms into 1/2-inch thick slices, set aside. Heat a medium skillet, add butter, pancetta and shallots and cook over medium heat until pancetta renders most of its fat and shallots are softened. Remove shallot mixture from pan and cool. in a large bowl, add mushrooms and shallot mixture together with the herbs, lemon juice and olive oil, season to taste with salt and freshly ground black pepper.
Fold a full sheet of parchment paper in half, cut an elongated heart shape using the entire width of the paper. Add 1/6th of the mushroom mixture to the bottom side of the parchment. Beginning at the center of the heart, begin making well creased folds, continue around the heart until package is completely enclosed. Bake just before serving in 450 oven about 5 minutes or until package has puffed up and is golden brown in color. Open in front of your guests with small scissors or paring knife.