Make and freeze the pecan- and dried fruit-studded cornbread up to a month in advance, and all you have to do on Thanksgiving is add a few more ingredients and bake.
Preheat oven to 400 degrees with rack in top third. Melt butter in an 8- or 9-inch-square baking dish in oven. Meanwhile, whisk together cornmeal, flour, sugar, baking powder, baking soda, salt, pecans, raisins, and cranberries in a bowl. Whisk together buttermilk and eggs in a large bowl. Gradually whisk melted butter (set aside baking dish) into buttermilk mixture.
Whisk cornmeal mixture into buttermilk mixture until just combined, then pour into buttered dish. Bake cornbread until golden and a toothpick inserted in center comes out clean, about 35 minutes. Let cool completely in pan on a wire rack, about 1 hour. Cornbread can be made 2 days ahead and stored at room temperature or frozen up to 1 month.
Preheat oven to 350 degrees. Cut cornbread into roughly 1-inch pieces and place on a rimmed baking sheet, along with any crumbs. Toast until golden, about 30 minutes.
Increase oven temperature to 400 degrees. Butter a 9-by-13-inch baking dish. Whisk together milk, cream, eggs, scallions, salt, and pepper in a large bowl. Fold in toasted cornbread and transfer to baking dish. Bake until custard is set and top is browned, 35 to 40 minutes. Let cool 15 minutes before serving.