Step #1 In a small bowl, soy sauce, mix hoisin sauce, & Chinese five spice, vinegar, sugar, if using.
Step #2 Rinse pork & pat dry; place this in a heavy 1-gallon zip-lock plastic bag.
Step #3 Add half the hoisin mixture, seal bag, & carefully rotate to coat pork with marinade.
Step #4 Chill at least 1 hr or up to 1 day.
Step #5 Cover up & chill remaining marinade.
Step #6 Lift pork from marinade (discard used marinade) & set on a rack in a foil wrap-lined 9- by 13-inch baking pan.
Step #7 Bake in a 425° regular or convection oven for 20 mins.
Step #8 Baste with reserved marinade & continue roasting, until a thermometer inserted into center of tenderloin reaches 155°, basting every once in awhile, 20 to 30 mins longer.
Step #9 Let rest at least 5 mins.
Step #10 Slice pork thinly across the grain & arrange on a platter.
Step #11 Garnish with cilantro sprigs, if using.
Step #12 Serve warm or cool, with hot mustard for dipping.
Step #13 Hot Mustard: In a small bowl, mix 3 tbsps dry mustard with 2 tbsps cold water.