This overnight recipe couldn't be easier - it takes only a few minutes to throw together the night before and all you need to do is cook it in the morning! It's best if you can cut your bread into slices at least a few hours before making the custard, but you can also dry them in a low oven if you forget this step.
The night before you want to cook your French toast, cut the challah (or brioche) into 2½cm (1 in) slices. Allow to dry out, uncovered, for a few hours (see note 1).
Add the eggs to a large bowl and whisk just to break them up a bit. Pour over the milk, cream, sugar, vanilla, nutmeg (if using) and salt. Whisk until thoroughly combined and no streaks of egg remain.
Pour the custard into a large, rimmed baking tray (large enough to accommodate all of your bread slices). Lay the bread slices on top of the custard, then flip them over to ensure that both sides have been coated. Cover the tray with plastic, move it to the refrigerator and allow it to chill overnight.
When ready to cook, remove the bread from the fridge. Set a griddle or skillet (preferably cast iron) over medium heat. Add a pat of butter. Once melted and foamy, add the soaked bread and cook until crisp and golden on one side - about 5 minutes. Flip and repeat on the other side. Serve immediately, topping with warm maple syrup.
Notes
If you forget to let your bread dry out, don't worry! Set a wire rack over rimmed baking sheet and lay the bread slices over it. Move it to a 100°C/200°F oven for 5 minutes before flipping and toasting for another 5 minutes. Proceed with the recipe as written.