Start by peeling and cubing the sweet potatoes into evenly sized pieces to ensure uniform cooking. Place them in a large pot and cover with cold water. Bring to a boil and cook for about 15-20 minutes or until the sweet potatoes are fork-tender. Drain well and transfer to a large mixing bowl.
Add the softened butter, granulated sugar, milk, vanilla extract, cinnamon, and nutmeg to the sweet potatoes. Mash everything together until smooth and creamy. You can use a potato masher or an electric mixer for a silkier texture. Taste and adjust sweetness or spices if desired.
In a separate bowl, combine the chopped pecans, brown sugar, and all-purpose flour. Pour in the melted butter and stir until the mixture is crumbly and holds together when pressed. If using, fold in the crumbled turkey bacon now for a savory contrast.
Spoon the mashed sweet potato mixture into your baking dish, smoothing the top with a spatula. Sprinkle the pecan streusel evenly over the surface, making sure to cover every inch for maximum crunch.
Preheat your oven to 350°F (175°C). Place the assembled casserole in the oven and bake for 25-30 minutes or until the streusel topping is golden brown and crisp. The sweet potato filling should be hot and slightly bubbling around the edges.
Let the casserole cool for 5-10 minutes before serving. This will help the topping set slightly and make it easier to scoop out portions. Enjoy your Sweet Potato Casserole with Pecan Streusel as a side dish or a comforting dessert.