Pork Tenderloin with Mushroom Stuffing and Pan-Seared Onion and Apples

STUFFING: In large skillet, heat butter and oil over medium-high heat; cook mushrooms and onions, stirring occasionally, until liquid is evaporated, about 8 minutes. Add apple and carrot; cook for 1 minute. Remove from heat. Add bread crumbs, parsley, thyme, sage, salt and pepper; mix well. Set aside.

Cut each tenderloin lengthwise halfway through; open like book. Place between plastic wrap or waxed paper; using meat pounder or rolling pin, pound to generous 1/4-inch thickness.

Cut about 7 pieces of kitchen string into 15-inch lengths; arrange crosswise about 1 1/2 inches apart in lightly greased roasting pan. Place 1 piece of meat on 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 ends under and press firmly in place. Top with remaining stuffing and meat.

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 in 400°F oven; reduce heat to 350°F and roast until meat thermometer registers 160°F, 1 1/4 to 1 3/4 hours. Transfer to cutting board; tent with foil. Let stand for 10 minutes. (Make-ahead: Let cool, cover and refrigerate for up to 24 hours.)

Slice roast and arrange on platter; surround with Pan-Seared Onion and Apples.

Cut uncored apples crosswise into scant 1/2-inch thick slices. Discard ends. Repeat with onion. In large skillet, heat half of the oil over medium-high heat; sear apples and onion until browned, 1 to 2 minutes per side, adding more oil if necessary.