01 - Preheat the oven to 350°F. Grease a 9x13-inch baking dish with 1 tablespoon unsalted butter, coating the bottom and sides evenly.
02 - Spread the bread cubes in an even layer in the prepared dish. Distribute the raisins uniformly over the bread surface.
03 - Whisk together the eggs, whole milk, heavy cream, granulated sugar, vanilla extract, cinnamon, nutmeg, and salt in a large bowl until completely combined and the sugar has dissolved.
04 - Pour the custard mixture evenly over the bread and raisins. Press down gently with a spatula to ensure all bread pieces are submerged in the liquid. Allow to rest for 10 minutes so the bread fully absorbs the custard.
05 - Bake for 40 to 45 minutes until the custard is completely set and the top is golden brown. A knife inserted near the center should come out clean.
06 - Remove from the oven and let cool for 10 to 15 minutes before serving. Serve warm, optionally dusted with powdered sugar or accompanied by vanilla sauce, whipped cream, or ice cream.