Melt butter in a large skillet over medium-high heat; add mushrooms and onion, and sauté 8 minutes or until tender. Stir in pecans, and set aside.
Place pork between 2 sheets of plastic wrap; flatten to 1/4-inch thickness, using a meat mallet or rolling pin. Sprinkle with salt and ground pepper.
Spread mushroom mixture evenly on 1 side of each tenderloin, leaving a 1/4-inch border. Roll up jellyroll fashion, starting with 1 long end. Wrap 4 bacon slices around each tenderloin, and secure with wooden picks. Place, seam sides down, on a lightly greased rack in a roasting pan. Rub evenly with sugar and cracked pepper.
Bake, uncovered, at 450° for 15 minutes. Reduce temperature to 400°.
Bake at 400° for 15 minutes or until a meat thermometer registers 160°.