This is a fabulous recipe that I adapted from the Canadian Living Magazine's October 2000 edition. I have made it often, and it always turns out beautifully, and offers the cook lots of great ideas for presentation.
I have used this stuffing in thick-cut pork chops, as well as a plain old boneless roast.
Heat butter with oil over medium-high heat; cook mushrooms and onions, stirring occasionally, for about 8 minutes or until liquid has evaporated. Add apple and carrot, cook for 1 minute more. Remove from heat. Add bread crumbs, parsley, thyme, sage, salt and pepper.
Cut each tenderloin lengthwise halfway through, and open like a book. Place between plastic wrap and pound to generous 1/4 inch thickness.
Prepare a roasting pan by spraying generously with cooking spray. Place a rack inside the roasting pan and spray that too.
Cut about 7 pieces of butcher's twine into 15-inch lengths, arrange cross-wise about 1 1/2 inches apart on the rack. Place one piece of meat on the strings. With hands, press half of the stuffing over meat to cover surface. Top with second piece of meat, placing wide end over thin end of first piece; tuck thin, pointy ends under and press firmly in place. Top with remaining stuffing, then the 3rd tenderloin, again, tucking the pointy end under.
Tie strings around roast, trimming any excess. Spread mustard all over roast. Arrange apple slices in lengthwise row on top; sprinkle with salt and pepper. Arrange thyme sprigs over top.
Place roast in oven and immediately turn heat down to 350 degrees. Roast until meat thermometer reads 160 degrees; about 1 1/4 - 1 3/4 hours.
Transfer to cutting board, tent with foil and let stand 10 minutes or so.
With a very sharp knife, slice roast and arrange on platter.