It’s not a big deal making eggs and toast to feed one or two. It’s when you try it for a crowd that things start to get tricky. The eggs or the toast always wind up cold, which is bad enough on a weekday, and even worse on a holiday morning.

But turn those ingredients into a breakfast casserole and something magical ensues.

Instead of remaining separate, the bread soaks up the egg, becoming custardy in the center, and puffed and golden on top after baking. You can stir in all kinds of other ingredients to fancy it up — any combination of bacon, sausage, cheeses, herbs or vegetables will heighten its appeal.

Best of all, not only can all of this be done in advance, it should be.

Assembling the casserole the day before allows the bread to soften and absorb the egg mixture as it sits. Then, the next day, all you need to do is pop it in the oven, leaving the cook plenty of time for other morning pursuits — going for a run or taking a shower (or both), caffeinating, even opening presents while the casserole slowly bakes until burnished.

Although many breakfast casseroles lean on bacon and sausage for character and heft, this one is meatless and bright. It’s filled with leeks and spinach sautéed until wilted, the pan deglazed with winy vermouth for extra zip. Then, the vegetables are mixed into a simple egg and milk custard that’s gently suffused with nutmeg, lemon zest and a pinch of cayenne.

I also threw in some cheese, choosing Gruyere for its rich nuttiness, along with some Parmesan for pungency and depth. But you can substitute cheddar, Gouda, manchego, blue cheese, even soft goat cheese, which would make the center of the casserole even creamier and tangier.

And should someone at your table miss the bacon, you can lay some slices on a sheet pan and cook them in the same oven alongside the casserole. This way, the casserole stays vegetarian while the bacon stays crisp, rather than softening into the puddinglike center of the casserole.

Add a beverage or two (coffee, tea, mimosas), and you’ll be all set to serve a savory, festive and highly convenient meal no matter how early your guests are set to arrive.