Rub meat with the mustard powder and thyme to taste. Place in a nonreactive dish and add 1/2 cup sherry, 1/2 cup soy sauce, the garlic and the ginger. Coat the loin well, cover and refrigerate overnight.
Preheat oven to 325 degrees F (165 degrees C).
Bake in the preheated oven for 25 minutes per pound, or until the internal temperature reaches 145 degrees F (63 degrees C). Baste the pork occasionally while cooking.
Meanwhile, heat the apple jelly in a small saucepan over medium heat. Add 2 tablespoons sherry and 1 tablespoon soy sauce, stir well, reduce heat to low and let simmer. Pour over the tenderloin when it’s done.
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