Heat the oven to 375°F. Arrange sweet potatoes on a foil-lined baking sheet in the center of the oven rack and roast until very soft to the touch, about 45 minutes to 1 hour. Prepare streusel topping.

Spread the pecans on a baking sheet and bake for 5 minutes at 350 degrees F, until lightly toasted. Transfer the pecans to a work surface and coarsely chop them.

In a medium bowl combine sugar, flour, and salt and whisk until incorporated. Add the butter. Using a pastry blender or your fingers, cut butter into the mixture until the mixture is in pea-size pieces or smaller. Combine with pecans and old fashioned oats and toss with your fingers to mix.

Form small clumps of the mixture with your fingers. Cover and place in the refrigerator until ready to use.

Remove sweet potatoes from oven when done baking and allow to sit until cooled, about 20 minutes.

Butter coat a 13-by-9-inch baking dish with butter and up the sides of the dish and set it aside.

Combine bourbon, zest of an orange, nutmeg, measured butter, salt and cinnamon in a small saucepan over medium heat and cook. Stir occasionally until the butter has melted and the mixture has started to bubble, about 5 minutes; set aside.

Cut sweet potatoes in half lengthwise when cooled. Scrape the flesh out with a spoon and transfer it to a large bowl; discard the skins. Using a potato masher, mash the flesh until it’s almost smooth but some chunks remain. Combine bourbon saucepan mixture with milk and stir until evenly combined. Pour mixture into prepared baking dish.

Evenly top the dish with streusel topping over the sweet potatoes. Return to bake until the streusel top is golden brown, about 20 minutes. Place the dish on a wire rack and let it sit 10 minutes before serving.