Grease a 9x13 inch baking dish generously with butter or non-stick spray.
Arrange half of the bread cubes in an even layer at the bottom of the dish.
Scatter the diced apples evenly over the first layer of bread, then top with the remaining bread cubes.
Whisk together the eggs, milk, heavy cream, 1/2 cup brown sugar, vanilla, 1 teaspoon cinnamon, and nutmeg in a large bowl until smooth.
Pour the custard mixture evenly over the bread and apples. Gently press down with a spatula to ensure the bread soaks up the liquid.
Cover firmly with foil and refrigerate overnight, or for at least 4 hours.
Preheat your oven to 350°F (175°C). Remove the casserole from the fridge to take the chill off.
Mix the flour, 1/3 cup brown sugar, remaining 1/2 teaspoon cinnamon, and cold butter in a small bowl until crumbly to make the topping.
Sprinkle the streusel topping over the casserole just before putting it in the oven.
Bake for 45 to 55 minutes, or until the top is golden brown and the center is set (no longer liquid).
Serve warm with maple syrup or a dusting of powdered sugar.
