Preheat oven to 400 degrees F. and lightly grease a large baking dish. Season pork tenderloin with salt and pepper to taste and set aside.
In a small bowl, stir together 1/4 cup brown sugar, 1 tablespoon soy sauce, and minced garlic. Spread mixture over the top of the pork.
Press pecan pieces onto the brown sugar mixture on the pork. Bake uncovered in preheated oven for 20 minutes.
While pork is cooking, prepare the sauce: In a medium sauce pan over medium-high heat, combine remaining 1/4 cup brown sugar, 1 tablespoon soy sauce, pineapple juice, and Dijon mustard. Bring mixture to a boil, reduce to a simmer for 3 to 5 minutes, then remove from heat.
Slice pork, spoon sauce over the top, and serve. Serves 6.
