I made the mistake once of walking into the Jewish 2nd Ave Deli in New York City and ordering a Reuben sandwich. The waitress tilted her head to the side, raised an eyebrow, and waited patiently for my lapsed-Jewish brain to do some basic calculations. After a couple of awkward beats, I apologized and ordered a tongue on rye with mustard instead.
If Jewish dietary law had been at the forefront of my mind, I would have known not to ask in the first place: Observant Jews don't mix meat with dairy. That makes a Reuben—toasted rye bread stuffed with corned beef, a heap of warm sauerkraut, gooey layers of melted Swiss cheese, and a generous dose of Russian dressing—pure sacrilege...or, should I say, sacrilege that's purely delicious.
Plenty of less strict delis will make a Reuben for you, but I've been shocked at the poor quality served at some places, including one very famous NYC deli that I won't identify (aside from mentioning that it's located on the corner of Houston and Ludlow, which of course couldn't possibly be enough info for you to figure out which one I'm talking about). The Reuben is an easy sandwich; there's no reason to get it wrong. Make it at home, and you're guaranteed not to have such a problem.
The hardest part of making a good Reuben is getting good corned beef and Jewish rye. The rye actually isn't such a big deal—once buttered and toasted, even lesser loaves will taste just fine. Bad corned beef is a little harder to cover up, but even then, the Reuben is forgiving. We can thank the melted cheese, sauerkraut, and Russian dressing for that.
Still, try to get the best quality you can wherever you live. There are a couple of ways to go about that. You can order cooked corned beef online from a place like Zingerman's in Michigan, or settle for a high-quality supermarket brand from the deli. Even better, if you're more ambitious, you can make and cook it yourself by following this recipe through step 3. You could also opt for pastrami in place of the corned beef, which isn't the traditional meat in a Reuben, but it still tastes damned good.
Typically at delis, the meat will be steamed whole, then carved by hand while it's still hot. If you've got a very sharp carving knife and a practiced hand, you can use this method at home. Otherwise, you can slice the meat by hand while it's cold, or, easier still, ask them to slice it for you at the deli counter. So long as you reheat it properly (and we'll get to that shortly), even presliced corned beef will come out plenty tender and juicy. I like my corned beef moderately thick-sliced—a little bit thicker than what's shown in these photos, to be honest—but not as ridiculously thick as some delis-that-shall-not-be-named tend to favor.
A classic melt is often assembled and then heated through in a skillet, which works fine if you're making one or two at a time. But I find that heating each ingredient separately, assembling, and serving works just as well to melt the cheese, and makes it easier to do multiple sandwiches at once.
We have a grilled-cheese philosophy here at Serious Eats, which is that both sides of each slice of bread should be toasted to maximize the crunch against the soft melted cheese. In some cases, this rule works for melts, which is what a Reuben technically is. But with the Reuben, I prefer to revert to single-side toasting, because I like the inner surfaces of the bread in a Reuben to be slightly steamed and tender, the whole thing partially fusing together.
Doing that is as simple as brushing one side of each bread slice with butter, then toasting it butter side down in a cast iron skillet or on a griddle until golden. There might be some contention about whether to toast that bread before or after forming the sandwich, but we can all agree that toasting it in a skillet with butter instead of in a toaster is essential.
Meanwhile, the most important other thing to do is to get the kraut and corned beef heated through. With the kraut, I just stick it in a saucepan with a small amount of water and warm it up.
To heat the corned beef, I use an old restaurant trick, which is to seal the meat in a container or package with a little bit of water and stick it in a warm oven; aluminum foil works just dandy as the package. The water generates steam, which, trapped in the foil, helps keep the meat from drying out.
As soon as you're ready to assemble the sandwiches, just spread a healthy amount of Russian dressing on the untoasted sides of half the bread, pile the corned beef (dripped free of any liquid) on top of that, mound the sauerkraut on top of that, and then lay a big slice of Swiss cheese on the hot kraut. Spread another layer of Russian on the untoasted sides of the remaining bread, put more cheese on that, and throw all of it into the oven until the cheese is melted, which doesn't take more than a few minutes.
Then close the sandwiches. The double layer of Swiss cheese will fuse together, joining the top slice of bread with what's below it, just as it should be.
Just remember not to serve it to any friends who keep kosher.