Preheat oven to 400F (200C). Spray a baking dish with cooking spray.
In a small bowl, stir together mustard, brown sugar, olive oil, garlic, and thyme.
Place sliced onion in a single layer in bottom of prepared dish.
Season pork with salt and pepper on all sides, and then place on top of onion in the dish.
Brush half of the mustard mixture over the pork.
Cover with foil and bake for 25 minutes.
Remove cover, brush with remaining mustard sauce, and put under the broiler for about 5 minutes (or until a crispy crust forms on top), and pork reaches an internal temperature of 140-150 degrees F.
Remove from oven and allow pork to rest for about 10 minutes before slicing and serving.