Our Test Kitchen has made dozens and dozens of breakfast casseroles over the years, but this one is our latest favorite. Made with day-old mini croissants, caramelized onions, and a rich and cheesy egg custard flavored with a touch of Dijon, this Croissant Breakfast Casserole is a real crowd-pleaser and can be made-ahead. Assemble the casserole and store it, unbaked, in the refrigerator overnight. Then, in the morning, all you have to do is pop it in the oven. Keep an eye on the casserole as it bakes. The buttery croissants exposed on top get dark quickly, so be sure to tent it with foil while it finishes cooking.