Lay pineapple rings and sliced onion in a 6-quart slow cooker. Rub pork loin with coriander, salt and pepper and place over them, fat-side up.
Whisk the reserved 1/2 cup pineapple juice with broth, brown sugar and lime juice and pour around the roast. Cover and cook on low for about 3 to 4 hours, checking after 2 1/2 hours, until the thickest center reaches 145°F.
Lift pork to a board and rest for 10 minutes. Transfer pineapple rings to a platter and pour onion and cooking liquid into a saucepan, skimming excess fat.
Simmer juices for 5 to 8 minutes until reduced to about 1 cup. Mix cornstarch with cold water and stir into the sauce for 2 minutes until lightly thickened.
Slice pork across the grain and serve with the softened pineapple rings, onion and a small spoonful of the fruit sauce.