Preheat oven to 375°F (190°C). Spread 1 cup (250 mL) of the soup on the bottom of 9 x 13-in. (23 x 32-cm.) baking dish.
Layer a third of the sweet potato slices, overlapping slightly, onto soup in baking dish. Next layer a third of the potato slices, overlapping slightly, followed by a third of onions, ½ cup (125 mL) soup then ½ cup (125 mL) cheese.
Repeat layers in step 2 twice more, finishing with cheese on the top. Cover dish with aluminum foil. Bake on middle rack, 50 min. to 1 hr. or until potato is tender when pierced with a sharp knife. Uncover and continue baking another 15 min., until cheese is melted and lightly golden.
Remove from oven and let set 5 min. before serving.