Add the brioche cubes to a large mixing bowl and set aside.
In a separate mixing bowl, whisk together the eggnog, eggs, granulated sugar, vanilla, cinnamon, and nutmeg until the custard is smooth.
Pour the custard mixture over the brioche cubes in the large mixing bowl. Use a spatula or your hands to gently toss until all the cubes are coated. Let the mixture sit for 15 to 20 minutes so the bread absorbs the custard.
Grease a 9x13 casserole dish and transfer the soaked brioche mixture into the dish. Spread it into an even layer.
In another bowl, combine the flour, brown sugar, cinnamon, and nutmeg. Add the melted butter and mix with a fork until it forms coarse crumbs.
Sprinkle the streusel topping evenly over the soaked brioche. Add a few fresh cranberries across the top.
Bake at 350℉ for 45-55 minutes until the top is golden and crisp and the center feels set when lightly pressed.
Cool for 10 minutes before slicing. Serve warm.