In a large skillet, saute the mushrooms, pepper and onions in butter until tender. Set aside.
Unfold pastry. Layer the ham, cheese and mushroom mixture off-center on each sheet of pastry. Fold pastry over filling; pinch seams to seal. Place in a greased 15x10x1-in. baking pan. Bake at 400° for 18-22 minutes or until golden brown. Let stand for 5 minutes. Cut each with a serrated knife into four slices. Freeze option: Allow mushroom mixture to cool before filling pastry. Cover and freeze unbaked filled pastries on waxed paper-lined baking sheets until firm. Transfer to resealable plastic freezer bags; return to freezer. To use, remove pastries from freezer 30 minutes before baking and place on a greased baking sheet. Bake and serve as directed.
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