I watched the following video for an "ultimate mac & cheese" months ago. It stuck with me, and luckily, I was able to find it again. Here it is, embedded for your delectation:
Most versions of macaroni and cheese use a roux (Béchamel, really; see comment) that gets fortified by more and more cheese that you stir into the mix. This recipe, which the chef says has been passed down in his family, avoids roux altogether in favor of a different binding or thickening agent: eggs. The chef stacks his macaroni and cheese in lasagna-like layers, then pours an eggs-and-milk mixture—which reminded me a bit of a nog—into the pot as a binder.
The mac & cheese is baked covered for about half an hour, then baked uncovered for another thirty minutes. The end result looks a lot like pizza. In that sense, this recipe is a bit heretical: it's recognizably mac & cheese, but it's mac & cheese that's been pulled in several different directions: toward pizza, toward egg nog (and possibly quiche or frittata), and toward lasagna. I love the unorthodox approach, though, and can't wait to try this sometime... but first, I need to purchase a sufficiently deep pot or baking dish.