Preheat oven to 400°F.
Peel and slice the Japanese sweet potatoes into ½-inch thick rounds.
Arrange the sweet potato slices in a baking dish.
Melt the butter and mix with the brown sugar, cinnamon, nutmeg, and salt.
Pour the butter mixture over the sweet potato slices and toss to coat.
Bake for 30-40 minutes, until the sweet potatoes are tender and caramelized.
