Preheat the oven to 375°F.
Combine brown sugar, fish sauce, vinegar, water, garlic, ginger, black pepper, and chiles in a small bowl. Whisk to combine and dissolve sugar and set aside.
Season tenderloin with salt and pepper. Heat oil in a large, oven-safe skillet over medium-high heat. Sear pork until golden brown on each side, about 2–3 minutes per side.
Turn off the heat and pour sauce over pork.
If the skillet is oven-safe, transfer to oven; if not, transfer pork to a foil-lined sheet pan. Bake for 15–20 minutes until internal temperature reads 145°F on a meat thermometer.
Remove from the oven and let rest for 5 minutes.
Slice, top with pan sauce, and garnish with fresh shallots, cilantro, and black pepper. Serve with white rice.
20 minutes | Serves 3 to 4
30 | Serves 4
minimum 9 hours | Serves 2