Preheat the oven to 400 degrees F. Peel the pears and cut them into thin wedges. Place them in a 9 X 13 inch baking dish. Then add the wine, melted butter, brown sugar, vanilla, cinnamon, and 1/2 teaspoon salt. Toss to coat the pears. Bake the pears for 20 minutes.
Meanwhile, mix brown sugar, Dijon mustard, 1 1/2 teaspoons salt, and 1/2 teaspoon of black pepper in a small bowl. Brush the mixture over the entire surface of the pork tenderloin.
Pour the chopped pecans in a shallow baking sheet or on a large cutting board. Roll the pork tenderloin in the nuts to coat on all sides.
Stir the pears after baking. Carefully lay the pork tenderloin on top of the pears. Press any runaway pecans back in place. Bake another 15-20 minutes, until the thickest part of the pork loin reaches between 145 F (with a light pink center) and 160 degrees F on a meat thermometer.
Allow the pork tenderloin to rest 3 minutes while the juices redistribute, then cut and serve with a side of baked pears.
Using a serrated knife to cut the pork loin will keep (most of) the pecans from falling off.