Start by prepping your sweet potatoes. Peel and cut them into large chunks.
Place them in a large pot and cover with water. Bring it to a boil over high heat and cook until the sweet potatoes are fork-tender, about 15-20 minutes. Once cooked, drain the water and transfer the sweet potatoes to a large mixing bowl.
Preheat your oven to 350°F (175°C). While the oven heats up, lightly grease a 2.5 to 3-quart baking dish or a 9×13-inch pan with non-stick cooking spray. Set it aside.
To the bowl with the cooked sweet potatoes, add the eggs, softened butter, milk, brown sugar, granulated sugar, vanilla extract, salt, and if you’re using them, the optional nutmeg and cinnamon. Using a handheld electric mixer, beat everything together on medium-high speed until the sweet potatoes are smooth, fluffy, and well-combined. You want a creamy texture with no large chunks of potato left. Pour the sweet potato filling into the greased baking dish, smoothing the top with a spatula so it’s even.
In a microwave-safe bowl, melt the butter by heating it on high for about 1 minute. Add the brown sugar, flour, and salt to the melted butter. Stir it together with a fork or your fingers until it forms a crumbly mixture.
Add in the pecan halves and toss to coat them evenly with the crumble mixture.
Sprinkle the butter pecan topping evenly over the sweet potato filling in your baking dish. Make sure you cover the entire top for that perfect balance of filling and crumble in every bite.
Place the casserole in the preheated oven and bake for about 45 minutes, or until the topping is golden brown and the filling is set around the edges.
If you’ve prepared the casserole in advance and it’s coming straight from the fridge, it may take closer to 50-55 minutes. Once the top is beautifully browned and the sweet potatoes are bubbling around the edges, it’s ready to come out of the oven.
Let the casserole cool for a few minutes before serving, but not too long—this dish is best served warm when the topping is still crunchy and the filling is soft and smooth.