Grease a 9x13-inch dish; spread bread evenly.
Whisk eggs, milk, cream, granulated sugar, vanilla, cinnamon, and nutmeg smooth in a large bowl.
Pour evenly over bread; press gently to soak. Cover and refrigerate overnight.
Heat oven to 350°F; mix butter and brown sugar in a small bowl.
Drizzle butter mixture over casserole.
Bake uncovered 45-50 minutes until golden and center set; rest 5 minutes.
Dust with powdered sugar; serve with warm syrup.
To freeze after step 3, cover tightly with plastic and foil; freeze up to 2 months. Thaw refrigerated overnight, then follow steps 4-7.