In A Large Resealable Plastic Bag Or Shallow Glass Container, Combine Lime Juice, Salt If Desired And Cayenne. Add Pork Chops And Onion; Turn To Coat. Cover And Refrigerate For At Least 2 Hours. Drain, Reserving Marinade And Onion. Grill Chops, Covered, Over Medium-Hot Heat For 8-10 Minutes On Each Side Or Until No Longer Pink. Place The Marinade And Onion In A Saucepan; Bring To A Rolling Boil. Serve With The Chops.