Preheat oven to 400°F (204°C). Line a rimmed baking sheet or large baking dish with foil and spray with cooking spray.
Pat the pork dry with paper towels and trim off any silver skin. Place in the prepared pan.
In a small bowl, stir together the mustard, brown sugar, salt, and pepper until the sugar dissolves completely and the glaze looks glossy.
Set aside 3 tablespoons of the glaze in a separate covered bowl to serve with the cooked pork. Keep it away from the raw meat.
Brush 2 to 3 tablespoons of the remaining glaze over all sides of the tenderloin.
Wrap the bacon around the pork, overlapping each slice slightly and tucking the ends underneath.
Bake uncovered for 20 minutes, or until the pork reaches about 135°F. Remove from the oven and baste with the pan juices.
Return to the oven and bake another 10 minutes, or until the pork reaches 145°F (63°C). If the bacon is not crisp enough, broil for 1 to 2 minutes, watching closely.
Tent with foil and rest at least 5 minutes before slicing into medallions. Serve with the reserved mustard sauce.