In a small bowl, mix hoisin sauce, vinegar, sugar, soy sauce, and Chinese five spice, if using. Rinse pork and pat dry; place in a heavy 1-gallon zip-lock plastic bag. Add half the hoisin mixture, seal bag, and gently rotate to coat pork with marinade. Chill at least 1 hour or up to 1 day. Cover and chill remaining marinade.
Lift pork from marinade (discard used marinade) and set on a rack in a foil-lined 9- by 13-inch baking pan. Bake in a 425° regular or convection oven for 20 minutes. Baste with reserved marinade and continue roasting, basting occasionally, until a thermometer inserted into center of tenderloin reaches 155°, 20 to 30 minutes longer. Let rest at least 5 minutes.
Slice pork thinly across the grain and arrange on a platter. Garnish with cilantro sprigs, if using. Serve warm or cool, with hot mustard for dipping.
Hot Mustard: In a small bowl, mix 3 tablespoons dry mustard with 2 tablespoons cold water. Let stand about 5 minutes. Stir in1 teaspoon Asian sesame oil. Makes about 3 tablespoons.