Preheat oven to 400. Line a baking sheet with aluminum foil (for easy clean-up) and spray with cooking spray.
Season pork tenderloin with salt, pepper and thyme and place on baking sheet.
Roast at 400 for 22-25 minutes. (The exact timing will depend on the size of your tenderloin and how done you like it. Use an internal thermometer.)
Meanwhile, make the balsamic reduction. Place balsamic vinegar in a small sauce pan over high heat. Bring to a boil, reduce heat to low and simmer for about 5 minutes, until the mixture is reduced and thickened.
Brush about half of the balsamic mixture onto the pork in the last minute of cooking. Reserve the rest for serving. (Tip: Keep the mixture on low and stir occasionally or transfer it to a bowl so it doesn’t stick and harden on the pan.)
After the pork has rested, slice and serve with remaining balsamic glaze.