Prepare a 9"x13" baking dish by coating the entire inside of the dish with butter or cooking spray. Set aside.
Slice the brioche bread, about 1-inch thick. Arrange slices in your prepared baking dish in rows, overlapping the slices. Set aside.
In a large bowl, whisk eggs. Add half-and-half, milk, sugar, vanilla, cinnamon, nutmeg, and salt. Whisk until well combined.
Pour the egg mixture over the bread slices, making sure they are covered evenly. Spoon some of the mixture in between the slices making sure all the slices are completely covered. Cover with foil and refrigerate overnight.
The NEXT DAY, preheat oven to 350°F. Meanwhile, carefully flip each piece of bread over to make sure everything is nice and coated with the mixture.
Combine all ingredients in a medium bowl and mix to combine. Spoon the topping on top of the prepared bread slices. Then spread evenly over the bread (like you are icing a cake). You can also spread some of the mixture in between the slices to make sure the entire slice is covered (if you prefer).
Bake for 35-40 minutes (no need to cover with foil), until puffed and lightly golden.