Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking dish with butter and arrange the bread cubes in an even layer.
In a large mixing bowl, whisk together the milk, heavy cream, eggs, granulated sugar, vanilla extract, cinnamon, nutmeg, and salt until smooth and well combined.
Pour the melted butter over the bread cubes and toss gently to coat. Then pour the custard mixture evenly over the bread, pressing down gently with a spatula to ensure all pieces are moistened.
Let the bread soak for 30 minutes at room temperature, pressing down occasionally so the custard absorbs fully. For best results, cover and refrigerate overnight.
Dot the top with small pieces of butter and sprinkle with turbinado sugar if using. This creates a beautiful caramelized crust.
Bake for 45-55 minutes until the top is golden brown and puffed, and a knife inserted in the center comes out mostly clean with just a few moist crumbs.
Remove from the oven and let cool for 10-15 minutes before serving. The custard will continue to set as it cools slightly.
