Heat the canola oil in large deep skillet or Dutch oven and brown the turkey breasts on both sides, starting skin-side down. Cool the turkey breasts; wrap and refrigerate to firm them back up.
Melt the butter in the skillet or pot you browned the turkey in and add the apples, celery and onions. Season with salt, pepper, bay and thyme; cook to tender for 10 minutes or so. Add the parsley and deglaze with Calvados or brandy. Add the sausage and the turkey trimmings and lightly brown; do not cook the sausage until dry. Stir in the stuffing, then season with a little celery salt and moisten with stock, adding just enough to make bread damp but not wet. Stir in the chestnuts. Cool the stuffing completely.
Roll out one sheet of puff pastry on lightly floured surface. Top the pastry with half of the the stuffing blend, leaving a border to work with. Place a browned breast skin-side down on the edge closest to you, then wrap and roll the Wellington, trim and tuck the pastry and secure the edges with a brush of water. Create any design you like, or simply brush with egg wash. Repeat with the remaining breast and chill for at least 30 minutes.
Arrange a rack at the center of the oven and arrange the breasts on a large, parchment-lined baking sheet with a few inches between them. Top with foil but do not seal; bake at 375° for 40 minutes. Remove the foil and bake 20 minutes longer, or until cooked though and deeply golden in color. Cool for 20 minutes before slicing and serving.
Serve with homemade cranberry sauce (mix 1 cup water, 1 cup sugar, 1 12-ounce bag cranberries; boil until popped and sauce forms) and gravy (butter and flour to thicken turkey stock; add pepper and Worcestershire, optional tempered egg yolk) and steamed green beans alongside.