Pat the pork tenderloin dry. Trim off any silver skin using a sharp knife, sliding the blade just under the tough membrane and pulling it away. Slice the tenderloin crosswise into medallions about 1 inch thick. Season all cut sides with salt, pepper, and garlic powder.

In a small bowl, whisk together the honey, soy sauce, rice vinegar, and minced garlic. In a separate small bowl, stir the cornstarch into the water until dissolved. Set both aside.

Heat a cast iron or heavy skillet over medium-high heat until it is very hot, about 2 minutes. Add the olive oil and swirl to coat. Place the medallions in a single layer without touching. Do not move them for 2 to 3 minutes. They should release easily from the pan and have a deep golden-brown crust. Flip each one with tongs and cook another 2 to 3 minutes on the second side.

Check the internal temperature with an instant-read thermometer. You are looking for 145 degrees F. Remove the medallions to a plate and let them rest for 3 minutes while you make the sauce. They will continue cooking slightly as they rest.

Reduce heat to medium. Add the butter to the same skillet and let it melt. Pour in the honey soy mixture and stir, scraping up any browned bits from the bottom of the pan. Those bits are flavor, do not leave them behind. Let the sauce bubble for 1 minute.

Stir the cornstarch-water mixture once more and pour it into the sauce. Stir constantly for about 30 to 60 seconds until the sauce thickens and turns glossy. It should coat the back of a spoon.

Return the pork medallions to the skillet and turn each one to coat in the sauce. Spoon extra sauce over the top. Garnish with sliced green onion and serve immediately over rice or mashed potatoes.