Preheat your oven to 350°F (175°C).
Grease a 9x13-inch baking dish with butter.
Layer half of the sliced sweet potatoes in the dish evenly.
Sprinkle half of the brown sugar mixture (combine sugar, cinnamon, nutmeg, and salt) over the sweet potatoes.
Pour half of the heavy cream over the layer of sweet potatoes and sugar.
Repeat the layering process with the remaining sweet potatoes, brown sugar mixture, and cream.
Cover the dish with foil and bake for 50 minutes.
Remove the foil and bake for an additional 20-25 minutes until the top is golden and bubbly.
Let the dish cool slightly before serving.