Preheat your oven to 375°F (190°C).
Grease a 9x13 inch baking dish with butter.
Layer half of the sweet potato slices in the bottom of the baking dish.
Sprinkle half of the brown sugar, cinnamon, nutmeg, and salt over the first layer.
Repeat with the remaining sweet potato slices and spices.
Pour the heavy cream evenly over the layered sweet potatoes.
Cover the baking dish with foil and bake for 30 minutes.
Remove the foil and bake for an additional 30 minutes until golden and bubbly.
Let it sit for 10 minutes before serving.