Preheat the oven to 375°F. Spray 13x9-inch baking dish with nonstick spray.
Place the potatoes in a Dutch oven, and cover with water. Bring to a boil. Reduce heat; simmer for 12 minutes or until tender. Drain.
Combine the half and half, syrup, vanilla, salt and egg in a large bowl, stirring with a whisk. Add the potato to the egg mixture; beat with a mixer at medium speed until smooth. Spoon the potato mixture into the prepared pan.
Combine the flour and sugar in the bowl of a food processor; pulse to combine. Add the chilled butter; pulse until the mixture resembles coarse meal. Stir in the pecans; sprinkle over the potato mixture.
Cover and bake for 15 minutes. Uncover and bake an additional 25 minutes or until the topping is browned and the potatoes are thoroughly heated.
Notes
If you do not have a food processor, use a pastry blender to cut the butter into the flour/sugar mixture until the mixture resembles coarse meal.
To make ahead: Assemble the potato mixture and sprinkle with the streusel, then freeze the casserole up to 2 weeks ahead of Thanksgiving. Thaw in the refrigerator and bake as directed. Or cook just the potatoes a day ahead, mash and refrigerate and proceed with the recipe as directed (starting with the addition of half and half) the day you're serving the dish.