Use a 6-quart slow cooker. Lay a length of foil on your countertop and place the fish directly in the middle. Dribble balsamic vinegar and honey over the top of each fillet, and place a handful of drained mandarin oranges on top.
Fold over foil and crimp the edges to form a packet. Put the foil packet into your slow cooker, and put the lid on (if you'd prefer to have one fish fillet per packet for serving, divide the sauce and the oranges). Cook on high for two hours, or until fish flakes easily with a fork. If your fish was frozen, it may take another 30 minutes or so, but you should still check it after two hours.