For the casserole: Adjust two oven racks to the upper and lower-middle positions and heat oven to 325 degrees. Arrange the bread in a single layer on two baking sheets. Bake until dry and lightly golden, about 25 minutes, rotating the baking sheets front to back and top to bottom halfway through the baking time. Allow the bread to cool
Grease the bottom and sides of a 9x13 baking dish with butter. Layer the dried bread tightly in the prepared dish, about two layers. Whisk together the eggs, milk, half-and-half, sugar, vanilla, cinnamon, and nutmeg and pour over the bread. Press lightly on the bread to submerge. Cover with plastic wrap and refrigerate for 8-24 hours.
For the topping: When ready to bake, adjust oven rack to the middle position and heat oven to 350 degrees. Mix the butter, brown sugar, and corn syrup together until smooth, then stir in pecans
To assemble and bake: Remove the casserole from the refrigerator and unwrap. Spoon the pecan topping over the casserole, then spread it evenly over the bread mixture. Place the baking dish on a rimmed baking sheet and bake until puffed and golden, about 60 minutes. Cool for 10 minutes before serving.