Preheat the oven to 350°F, and grease a 2.5 quart baking dish or 11 x 7-inch pan. Set aside.
Bring a large pot of water to a boil, and add the sweet potatoes. Cook until they are soft and fork tender, about 20 to 25 minutes. Drain and transfer to a large bowl to cool.
3 pounds sweet potatoes
While the potatoes cool, make the topping. In a medium bowl, combine the melted butter, brown sugar, flour, and cinnamon. The mixture will be crumbly. Stir in the pecans, then set aside.
5 tablespoons unsalted butter, ⅔ cup light brown sugar, ½ cup all-purpose flour, ¼ teaspoon ground cinnamon, 1 cup pecans
Use an electric mixer to beat the sweet potatoes until smooth. Add the brown sugar, butter, salt, cinnamon, and nutmeg. Mix until just combined, then add the beaten eggs. Continue to mix until the eggs are incorporated.
½ cup light brown sugar, ¼ cup unsalted butter, ½ teaspoon salt, ¼ teaspoon ground cinnamon, ⅛ teaspoon ground nutmeg, 2 large eggs
Spread the sweet potato mixture into the prepared baking dish, and top with the streusel topping. Bake for 40 to 45 minutes until the top is golden brown and the center is set.
Let the casserole cool for 5 to 10 minutes before serving. Store any leftovers in the refrigerator for up to 3 days.