Adjust 2 oven racks to upper middle and lower middle positions and heat oven to 325 degrees. Spread bread out over 2 baking sheets and bake until dry and light golden brown, about 25 minutes, switching and rotating baking sheets halfway through baking time. Let bread cool completely.
Coat a 13 x 9 inch baking dish with the tablespoon of butter and pack dried bread into dish. Whisk eggs in large bowl and then whisk in milk, cream and sugar, vanilla, cinnamon and nutmeg. Pour egg mixture evenly over bread and press on bread lightly to submerge.
For the topping: Stir butter, brown sugar, and pancake syrup together until smooth, then stir in pecans.
To store: Transfer topping to airtight container and wrap dish tightly with plastic wrap. Refrigerate topping and casserole separately for at least 8 hours or up to 24 hours.
To serve: Adjust oven rack to middle position and heat oven to 350 degrees. Unwrap casserole and sprinkle topping evenly over top, breaking apart any large pieces with your fingers. Place casserole on rimmed baking sheet and bake until puffed and golden, about 1 hour.
Serve at once
