Grease a 9x13-inch baking dish with butter or cooking spray.
Layer half of the bread cubes in the bottom of the dish, then top with diced apples, followed by remaining bread cubes.
In a large bowl, whisk together eggs, milk, heavy cream, granulated sugar, brown sugar, vanilla, cinnamon, nutmeg, and salt until well combined.
Pour the egg mixture evenly over the bread and apples, making sure to soak all the bread. Press down gently with a spatula.
Drizzle melted butter over the top and cover with plastic wrap. Refrigerate for at least 2 hours or overnight.
When ready to bake, preheat oven to 350°F. Remove plastic wrap and bake for 45-50 minutes until golden and set in the center.
Let cool for 10 minutes before serving. Dust with powdered sugar and serve with maple syrup if desired.
To freeze: Wrap cooled casserole tightly in plastic wrap, then aluminum foil. Freeze for up to 3 months. Thaw overnight in refrigerator and reheat at 350°F for 20-25 minutes.