Preheat the oven to 350°F and set an oven rack in the middle position.

Bring a large pot of water to a boil (no need to salt). Add the sweet potatoes and cook until soft and very tender, 20 to 25 minutes. Drain and let cool.

Meanwhile, make the topping: Melt the butter in a medium bowl in the microwave. Add the brown sugar, flour, and cinnamon and mix until well combined. It should look a little clumpy. Stir in the chopped nuts and set aside.

In a large bowl, mash the cooked and drained sweet potatoes with a potato masher or large fork until very smooth. Add the melted butter, salt, sugar, and nutmeg and mix well. Mix in the eggs. It's okay if there are some lumps. Set aside.

Transfer the sweet potato mixture to a 2-quart baking dish and spread evenly. Sprinkle the topping evenly over the sweet potato mixture and bake, uncovered, for about 40 minutes, or until the topping is nicely browned. Let cool slightly before serving.

The assembled casserole can be refrigerated for 2 days or frozen for up to 3 months. If frozen, thaw overnight in the fridge and proceed with baking instructions.