Prepare the Sweet Potato Filling

1. Cook the Sweet Potatoes: Peel and dice the sweet potatoes, then boil them in a large pot of salted water for about 15-20 minutes, or until fork-tender. Drain and transfer to a large mixing bowl.
2. Mash the Potatoes: Use a potato masher or electric mixer to mash the sweet potatoes until smooth.
3. Mix Ingredients: Add melted butter, brown sugar, heavy cream, eggs, vanilla extract, cinnamon, nutmeg, and salt. Mix until well combined and creamy.

Assemble the Casserole

1. Preheat Oven: Preheat your oven to 350°F (175°C). Grease a 9×13-inch baking dish.
2. Spread Filling: Pour the sweet potato mixture into the prepared baking dish and smooth it into an even layer.

Make the Topping

1. Combine Topping Ingredients: In a medium bowl, mix together brown sugar, flour, and softened butter until crumbly. Stir in the chopped pecans.
2. Add Topping: Sprinkle the topping evenly over the sweet potato mixture.

Bake the Casserole

1. Bake: Place the dish in the preheated oven and bake for 35-40 minutes, or until the topping is golden and the casserole is heated through.
2. Cool Slightly: Allow the casserole to cool for about 10 minutes before serving.