I first knew flap meat by its local New England name of sirloin tip. Go to any old school dive or tavern with a menu, and you're bound to run into them, cut into cubes, stuck on a skewer, and grilled over an open coal fire, just like they do at Santarpio's over in East Boston. When grilled right, they're tender, juicy, takes on marinades extremely well, and have a robust beefy flavor that a lot of other cuts-for-kebabs lack. That and they're cheap. Not just "cheaper-than-tenderloin-but-still-kinda-expensive" cheap, but actually cheap.
It wasn't until I moved back to New York that I realized that nobody outside of New England knows what sirloin tip is, and it wasn't until even later that I realized that the "faux hanger" and "flap meat" that the butchers around here sell are in fact the exact same cut of beef, just left whole rather than sliced into tips.
Of all the inexpensive cuts of beef, it's one of the most versatile. It takes great to fast-cooking methods like grilling or searing. It's excellent cooked whole and sliced into thin strips. It can't be beat cubed and put on skewers. It has a coarse texture that grabs onto marinades and seasonings. It's even great as a slow-cooked braise, where if comes apart into tender shreds, like a Cuban ropa vieja.
Here's how to deal with it.
Also Sold As: Faux hanger, bavette (France), sirloin tip (New England).
Where It's Cut From: The bottom sirloin butt—the same general region where the tri-tip comes from.
Flap meat comes in a few forms depending on where you live, but it's pretty much always delivered to the supermarket or butcher as a whole cut of meat, so if you live in an area where selling it in strips or cubes is the norm (such as in New England), instead ask the butcher to give you a whole trimmed flap steak. This gives you more options when you get it home.
As a relatively lean cut, there's not really any need to spring for Prime-graded flap steak. The Choice stuff will taste just as good and costs less.
More than any other cut I know of, flap meat is pretty terrible when it's cooked rare. You can feel it for yourself when it's still raw: this is some mushy-a*s meat. Only by cooking it to medium-rare or medium can you get it firm enough to not squish around in your mouth as you chew it.
Flap meat doesn't require the extreme heat of a skirt steak, and it doesn't have the fat flareup problems of a short rib, which make sit pretty simple to cook. Just build a hot fire (I use a single chimney), build it up on one side of the grill, the lay on the flap (after seasoning it, of course). Cook it by flipping every minute or so until it gets to at least 125°F at its thickest part
If it ever threatens to start burning on the exterior before the center is done, you can slide it on over to the cooler side of the grill for some more gently cooking. As with all meat, it benefits from a few minutes of resting before you slice into it.
Slicing and Serving
Flap meat has an extremely coarse grain with an obvious direction. It runs all the way down the steak crosswise. This makes it hard to cut against the grain into thin, bite-sized pieces (you'd end up with strips sliced lengthwise from the meat).
Instead, the best thing to do it first divide it into three or four pieces, slicing with the grain, then to rotate each of those pieces 90 degrees and slicing them thinly against the grain.
Since its shape, thickness, and proclivity for marinades makes it similar to flank steak, you can use it pretty much interchangeably. Think of it as flank steak's tastier, sexier cousin.