Slice the pork tenderloin into medallions, pat dry with paper towels, and season lightly with salt and pepper.

In a small bowl, whisk together the soy sauce, brown sugar, rice vinegar, and water until the sugar begins to dissolve.

Heat the vegetable oil in a large skillet over medium-high heat. Add the pork slices in a single layer and sear for 2-3 minutes per side until browned. Remove pork to a plate.

Reduce heat to medium. Add the minced garlic, ginger, and half of the sliced jalapenos to the skillet. Sauté for 1 minute until fragrant.

Pour the soy sauce mixture into the skillet, scraping up any browned bits. Bring to a simmer.

Whisk the cornstarch slurry and stir it into the simmering sauce. Cook for 1-2 minutes, stirring continuously, until the sauce becomes dark, glossy, and thick.

Return the cooked pork to the skillet. Toss the pork in the glaze for 1 minute until completely coated and heated through.

Remove from heat, stir in sesame oil if using, and top with the remaining fresh jalapeno slices. Serve immediately.