Grease a 9×13 baking dish with the tablespoon of unsalted butter.

Halve the croissants lengthwise with a serrated knife (presentation version) or tear them into rough two-inch chunks (rustic version). Arrange half in a single layer in the baking dish.

Whisk the eggs, brown sugar, vanilla, cinnamon, nutmeg, and salt together in a medium bowl until combined. Whisk in the heavy cream and milk until smooth.

Pour half the custard evenly over the bottom layer of croissants. Press them down gently with a spatula so they start to soak.

Add the remaining croissants on top. Pour the rest of the custard over everything, pressing again so no edges stick up dry.

Cover with foil and refrigerate at least 30 minutes, ideally overnight. If chilling overnight, take the dish out 30 minutes before baking to take the chill off.

Set the oven rack to the center position and preheat to 375°F.

Bake covered for 25 minutes. Uncover and bake another 20 to 25 minutes until the top is golden and the center is set (a knife inserted in the middle should come out with moist crumbs, not wet custard). If it browns too fast, tent loosely with foil.

Cool for 5 minutes. Finish with powdered sugar, maple syrup, fresh berries, or toasted nuts. Serve warm.