Spread sliced onion out on a greased rimmed baking sheet, place pork on top. Season with salt and pepper.
Cover with foil and roast at 325° F for 30 minutes.
Remove foil. Top roast with sliced orange and lime rounds. Continue roasting until meat has an internal temperature of 160° F.
Place pan under a broiler several minutes until roast is slightly charred. Rest 10 minutes before slicing. Serve with additional fresh sauce if desired.