Make the marinade: In a bowl, whisk soy sauce, pineapple juice, brown sugar (or honey), vinegar, garlic, ginger, sesame oil, and chili flakes or sriracha if using. Taste and adjust for sweetness and salt. You want it slightly sweet-salty with a gentle tang.

Prep the pork: Pat the tenderloins dry and season lightly with salt and pepper. Place them in a zipper bag or shallow dish.

Marinate: Pour two-thirds of the marinade over the pork. Reserve the remaining third in the fridge for later. Seal and refrigerate the pork for at least 30 minutes and up to 8 hours. Flip once or twice to coat evenly.

Preheat and prep: Heat the oven to 400°F (205°C). Set a large oven-safe skillet over medium-high heat and add the neutral oil. If grilling, preheat to medium-high and oil the grates.

Sear the pork: Remove pork from marinade and pat dry. Sear on all sides until browned, about 2–3 minutes per side. Do not crowd the pan. Discard the used marinade.

Add pineapple: Arrange pineapple rings or chunks around the pork in the skillet. Brush the pork with some of the reserved clean marinade.

Roast: Transfer the skillet to the oven. Roast 12–18 minutes, brushing once more with marinade halfway through. Cook until the thickest part reaches 140–145°F (60–63°C).

Rest the meat: Transfer pork to a cutting board and rest 5–10 minutes. This keeps the juices in the meat instead of on the board.

Glaze (optional but great): While the pork rests, simmer the remaining reserved marinade in the skillet with the pineapple over medium heat for 2–3 minutes. If you want it thicker, whisk cornstarch with 1 tablespoon water and stir in. Cook until glossy.

Slice and serve: Slice pork into 1/2-inch rounds. Spoon the pineapple and glaze over the top. Finish with green onions, sesame seeds, and a squeeze of lime if you like.