Preheat your oven to 375°F (190°C) and grease a 9x13 inch baking dish.
Place the peeled and cubed sweet potatoes in a large pot and cover with water. Bring to a boil and cook for 15-20 minutes, or until fork-tender. Drain well.
In a large mixing bowl, mash the cooked sweet potatoes. Add the melted butter, milk, eggs, 1/2 cup brown sugar, vanilla extract, and salt. Mix until smooth and creamy.
Spread the sweet potato mixture evenly into the prepared baking dish.
In a separate medium bowl, prepare the streusel topping by combining the flour, 1/2 cup brown sugar, chopped pecans, and rolled oats.
Pour the melted butter for the topping over the dry ingredients and stir until the mixture looks like coarse crumbs.
Sprinkle the pecan and oat streusel evenly over the sweet potato layer.
Bake in the preheated oven for 25-30 minutes, or until the topping is golden brown and the casserole is bubbly.
Let stand for at least 10 minutes before serving to allow the filling to set.