Season pork roast with coriander, salt and pepper. To the slow cooker, add orange juice, broth, fish sauce, garlic and ginger. Scatter half the onion and orange in the slow cooker, top with pork, then add remaining onion and orange on top of the pork. Cover and cook on low until an instant-read thermometer inserted in the center of the pork registers 160F, about 3-4 hours. Transfer the pork to a carving board. Rest for at least 15 minutes.