Prep. Preheat oven to 350°F. Grease a 9x13-inch baking dish with butter.
Dice brioche. Cut brioche into 1-inch cubes and spread evenly in prepared dish.
Make cream base. Whisk together milk, cream, eggs, both sugars, melted butter, vanilla, cinnamon, nutmeg, and salt until smooth.
Soak raisins. Soak raisins in warm water for 10 minutes, drain well, and sprinkle over bread cubes.
Assemble and rest. Pour custard mixture over bread. Press bread down gently to ensure all pieces are soaked. Let sit 10-15 minutes.
Bake covered. Cover loosely with foil and bake 30 minutes.
Baked uncovered. Remove foil and bake another 20-25 minutes until golden brown and center is set.
Cool and serve. Cool 10 minutes before serving.