Preheat oven to 400F (200C). Spray a baking dish with cooking spray.
In a small bowl, whisk together hoisin sauce, rice vinegar, honey, lime zest, and lime juice.
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 hoisin glaze over the pork.
Cover with foil and bake for 25 minutes.
Remove foil, brush with remaining glaze, 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.