The Roast Beef Cuts You Need For The Absolute Best Sandwiches
It's hardly a surprise that your lunchtime roast beef sandwich isn't prepared the same way as your dinnertime meal of roast beef and vegetables. Of course, one is typically served cold and the other hot, but it's more than that. For example, chuck steak works for braising, but you want less fatty cuts if you're planning to slice it into deli meat. Lean meats are nearly always the better choice for deli-style roast beef sandwiches, unless you like your sandwiches to be uncomfortably tough.
Specifically, you might consider a round cut, which are among the leanest cuts of beef you can buy. A bottom round comes from large muscles and can be evenly cut into large slices with a large surface area. Eye rounds will get you smaller slices with a similar leanness. However, some slightly fattier loin cuts such as top loins also work, because the fat is all gathered at the surface and it's easy to slice off so you can use the rest for sandwich slices. Just slice it off after cooking so that fat can baste the meat first.
Trim the fat
The reason for this involves the ways that marbled fat responds to heating (or lack thereof) and slicing. Lean meat is much easier to cut into thin slices without the tough, marbled streaks of fat getting in the way. The fat on a rich, marbled cut of beef is supposed to turn melty and tender when you heat it up, which is what makes fatty meat perfect for family-style roasts.
But when cooled down, that tenderness goes away and the marbling turns into gross, unchewable white lines in the meat. They're a chore to bite into, and very different from the iridescent "rainbow spots" on deli meats that sometimes show up, which come from light reflecting off the muscle fibers.
That said, your lean roast beef slice should still have a little chewiness. Roast beef shouldn't be soft. You'll often hear terms like "mineral" or "earthy" to describe the ideal taste of a lean roast beef deli slice. For a classic roast beef sandwich, lots of salt and horseradish are common additions.