Cut the challah or brioche into 1-inch cubes, and place them evenly in a generously greased 3-quart casserole dish.

In a large mixing bowl, whisk together the eggs, whole milk, heavy cream, granulated sugar, brown sugar, vanilla extract, ground cinnamon, and nutmeg until smooth.

Pour the custard mixture evenly over the cubed bread, ensuring every piece is thoroughly soaked. Cover the dish with plastic wrap and refrigerate for at least one hour or overnight.

Preheat your oven to 350°F (175°C).

In a separate bowl, mix together the flour, brown sugar, ground cinnamon, and salt. Work in the cold, cubed unsalted butter until crumbly.

Uncover the casserole, sprinkle with streusel topping, and bake for approximately 45-50 minutes until the top is golden brown and the custard is set.

Allow the casserole to cool for about 10-15 minutes before serving warm with a dusting of powdered sugar or fresh berries.