Grease a 9x13-inch dish with butter or spray.
Layer half the bread, apples, then remaining bread.
Whisk eggs, milk, cream, sugars, vanilla, cinnamon, nutmeg, and salt in a large bowl.
Pour evenly over filling; press gently with spatula to soak bread.
Drizzle with butter; cover with plastic and refrigerate at least 2 hours or overnight.
Heat oven to 350°F; remove plastic and bake 45-50 minutes until golden and center set.
Cool 10 minutes; serve with optional powdered sugar and syrup.
To freeze, wrap cooled casserole tightly in plastic then foil; freeze up to 3 months. Thaw refrigerated overnight; reheat at 350°F for 20-25 minutes.