Preheat the oven to 400°F (200°C).
Season the pork tenderloin with salt and pepper.
In a small saucepan, combine apple cider, Dijon mustard, brown sugar, and apple cider vinegar.
Bring the mixture to a simmer over medium heat, stirring occasionally until slightly thickened, about 10 minutes.
Stir in dried thyme, then remove from heat and set aside.
In an oven-safe skillet, sear the seasoned pork tenderloin for about 2-3 minutes on each side until browned.
Brush half of the glaze over the pork tenderloin.
Transfer the skillet to the preheated oven and roast for 20-25 minutes, or until the internal temperature reaches 145°F (63°C).
Once cooked, remove the pork from the oven and let it rest for 5-10 minutes before slicing.
Serve the remaining glaze on the side or drizzled over the sliced pork.