Trim the tenderloin: Pat the pork dry.
Trim away any silver skin with a sharp knife. Season all over with kosher salt and black pepper.
Mix the marinade: In a bowl, whisk 1/3 cup honey, 1/4 cup low-sodium soy sauce, 3–4 minced garlic cloves, 1 tablespoon rice vinegar, 1 teaspoon Dijon mustard, 1 tablespoon oil, and a pinch of red pepper flakes if you like heat.
Marinate: Place pork in a zip-top bag or shallow dish. Pour half the marinade over the pork and refrigerate 30 minutes to 4 hours.
Reserve the other half for the sauce.
Preheat and prep: Heat the oven to 400°F (205°C). Set a large oven-safe skillet (cast iron works well) over medium-high heat with a thin film of oil.
Sear: Remove pork from the marinade, shaking off excess. Sear the tenderloin on all sides until browned, about 2–3 minutes per side.
Don’t crowd the pan.
Roast: Transfer the skillet to the oven. Roast 12–18 minutes, depending on thickness, until the center reaches 140–145°F for a juicy, slightly pink interior.
Rest the meat: Move the pork to a cutting board and tent loosely with foil. Rest 8–10 minutes to keep it juicy.
Make the sauce: While the pork rests, pour the reserved marinade into the skillet.
Bring to a simmer over medium heat for 2–3 minutes. For a thicker glaze, whisk 1 teaspoon cornstarch with 1 tablespoon water and stir it in. Finish with 1 tablespoon butter for a glossy finish, if desired.
Taste and adjust with more honey, vinegar, or soy as needed.
Slice and serve: Slice pork into 1/2-inch medallions. Spoon the honey garlic sauce over the top. Garnish with chopped green onions or parsley and a touch of citrus zest if you like.