Whisk eggs, milk, cream, granulated sugar, vanilla, cinnamon, nutmeg, and salt until smooth.
Pour the custard over the bread and gently press to soak.
Let stand 15 to 30 minutes, or cover and chill overnight.
Preheat the oven to 350°F.
Stir melted butter with brown sugar until syrupy; drizzle over the casserole.
Cover with foil and bake for 25 minutes.
Uncover and bake 18 to 22 minutes more, until set and golden.
Cool 10 minutes. Dust with powdered sugar and serve with warm maple syrup.
Notes
For a pecan crunch, scatter 1/2 cup chopped pecans over the top before baking. To make ahead, assemble and refrigerate up to 12 hours; bake straight from the fridge, adding a few extra minutes if needed.