Unroll cresent rolls. Place 1 rectangle on baking sheet, sealing perforations. Sprinkle half the onion and mushrooms over dough; place cheese in center. Sprinkle with remaining onion and mushrooms. Fold up edges of dough to enclose cheese; seal edges. Place remaining portion of dough over top, folding edges down and sealing. Bake at 375 degrees for 20 to 25 minutes or until golden brown.