Unroll crescent dough; separate into 16 triangles. With wide ends of triangles towards the center, arrange 8 triangles in a circle on a baking stone. Corners of wide ends will touch and points will extend 1 inch beyond the edge of the baking stone. Arrange remaining 8 triangles in the center, matching wide ends. Seal seams using the rolling pin. (Points will overlap in the center. Do not seal.)
Place mayonnaise, mustard, and black pepper in a mixing bowl. Chop turkey, slice celery, and snip parsley. Add turkey, celery, parsley, and cranberries to mixing bowl. Grate cheese into the mixing bowl. Mix filling and then scoop over seams of dough, forming a circle.
Coarsely chop walnuts, sprinkle over the filling.
Beginning in the center, lift one dough triangle across the filling mixture. Continue alternating with outer triangles, slightly overlapping to form a wreath. Tuck last end under first.
Separate egg over small mixing bowl using an egg separator. (Discard yolk or set aside for another use.) Lightly beat egg white; brush over the dough using a basting brush.