Instructions

1. Combine the soy sauce, pineapple juice, 2 tablespoons of brown sugar (3 tablespoons for a sweeter sauce), mirin, sesame oil, ginger, and garlic in a small bowl and whisk to combine. 
2. Pour the mixture into a 1-gallon resealable bag. Add the pork tenderloin and seal, pushing as much air out as possible. Place in a shallow pan and refrigerate, turning occasionally, for a minimum of two hours or overnight. (I also inject some of the marinade into the pork using a meat syringe. This gets deeper flavor faster.) 
3. Pull the tenderloin from the refrigerator at least 30 minutes to an hour before cooking and let it warm on the counter. When ready to cook, preheat the grill on medium high, about 400°F. Once hot, scrape the grates clean using a wire brush. Oil the grates very generously to prevent meat from sticking.
4. Remove the pork from the bag, reserving the marinade, and place on the grill. Cover and cook, flipping the tenderloin only once, until internal temperature reaches 140°F (see Recipe Note), about 5 to 7 minutes per side. 
5. Meanwhile bring the reserved marinade to a boil in a small saucepan and cook until reduced by half, about 5 to 8 minutes. Brush the sauce over the tenderloin during the final minutes of cooking and serve the remainder on the side.
6. Remove the tenderloin from the heat and allow to rest for 10 minutes; the meat will finish cooking from residual heat and the juices will redistribute for optimal flavor. Slice crosswise into thin pieces before serving.