Trim the silverskin off the tenderloin — it won't break down and will toughen the meat. Pat dry with paper towels. Season all over with kosher salt, black pepper, and garlic powder. To marinate, combine soy sauce, honey, garlic, and mustard (or honey-Dijon-chili powder) in a sealed bag; marinate at least 1 hour, up to overnight in the fridge. Pull pork out 15 minutes before grilling and shake off excess marinade. Discard used marinade unless you boil it thoroughly first. Oil the grill grates lightly just before cooking.

Preheat grill to medium heat with two-zone setup: direct heat on one side, indirect on the other. Heat for at least 5 minutes, then clean and oil the grates. Sear the tenderloin over direct heat, turning every few minutes, until browned on all sides — about 10 to 15 minutes total. Move to indirect heat, close the lid, and cook until an instant-read thermometer reads 145°F in the thickest part. Total grill time is typically 16 to 24 minutes, possibly up to 30 minutes depending on grill temp and size. Do not rely on color alone — use a thermometer. If basting with barbecue sauce, apply during the indirect heat phase and turn once or twice to build a light glaze.

Pull the tenderloin at 145°F internal temp. Transfer to a cutting board, tent loosely with foil, and rest at least 10 minutes — this keeps the juices in the meat. Slice against the grain into ½-inch medallions. Interior should look slightly rosy. Don't go past 155°F or it dries out fast. Always use a thermometer — don't rely on time alone. Serve with boiled reserved marinade and your chosen sides.