Grease a 9x13-inch baking dish and spread half of the bread cubes evenly in the dish.
Layer the sliced peaches over the bread cubes.
Top with the remaining bread cubes, spreading them out evenly.
In a large bowl, whisk together the eggs, milk, heavy cream, granulated sugar, brown sugar, vanilla extract, cinnamon, nutmeg, and salt until well combined.
Pour the egg mixture evenly over the bread and peaches, ensuring all the bread is soaked.
Drizzle the melted butter over the top.
Cover the dish with plastic wrap and refrigerate for at least 2 hours, or overnight for best results.
Preheat the oven to 350°F (175°C).
Remove the plastic wrap and bake the casserole for 45 to 60 minutes, or until the top is golden brown and the center is set.
Allow the casserole to cool for about 10 minutes before serving.
Dust with powdered sugar and serve with maple syrup if desired.