Flat Meat?

Flap meat.  Flap, not flat.

Supermarkets also have mom and pop steaks  and london broil (chuck) of which there is no such thing. It is probably Flap. It could come from the top piece on th rib(deckles) or the chuck ,both of which are used for chopped mest.

He is most likely referring to what is called Chuck Tail Flat, which is the "butcher's" term for Boneless Short Ribs. It's not usually found in grocery stores. Costco Business Centers sell this in Select and Choice grades. There are 9 Costco Business Centers that I know of. Hopefully there's one near you.

Blessings.

So sorry for the lateness of this comment but I just found the site.  I actually went on the Test Kitchen webpage and they are saying sirloin tips or "flap" meat.  So most of the responses here are correct.  Like you I to heard "flat".  Hope that helps you and others.
 

I would guess that flap meat is the piece above the flank.  This piece has lots of flavor, but is not too tender.  Ideal for ground beef.

Flap Meat.....comes from the bottom sirloin, very flavorful. "meaty" taste.....looks similar to skirt but much larger, avg weight 5lbs. Nearly 100% yield... I end up with 1-2 lbs of waste from a 25lb average bag.

I use it a lot for steak sands, carne asada tacos etc.....simple marinade charbroil hot and fast not past MR.

I had the same question about the America's Test Kitchen "Pub Burgers".  I am so glad someone else asked and I am not the only one who heard "flat".  I went to the store where the guy behind the butcher counter looked at me like I was nuts and tried to sell me flat brisket, even though I told him I was trying to grind my own meat.  Funny.  I did not buy it.

I will try Costco like someone else suggested.

Thank you!

For hamburgers, I like to use a combination of chuck, brisket, and boneless short rib.  I find that this combination makes for a tender, rich beefy flavored burger....tomato ketchup optional.  .  

Best place to get flap meat is a Mexican market.

NOOOOOOOOOOOOOOOOO.  These guesses are soooooooooooooooooooooo wrong -- and not in the good way either.  

No.  And why guess? 

Unsurprisingly ChefBuba, and America's Test Kitchen website (h/t mdmphd) had it write from the giddyap. 

Flap DOES come from the bottom sirloin, and NOT from the rib or chuck.  If you want to check for yourself, it's IMPS 185A.  The IMPS number will help if you want to order flap from a custom butcher who isn't familiar with it's common name.  Meat cut names can be regional.  If a cut is labeled as something other than you'd expect in a different area that doesn't mean the retailer is acting in bad faith by labeling chuck as "London Broil" as opposed to London Broil which comes from the plate or flank.  The key is to get everyone on the same page. 

If you're serious about meat, you may want to put a site like Ask the Meatman in your bookmarks.  If you're serious about understanding where beef cuts come from on the animal, or just want to pretend you know what you're talking about, you should bookmark Bovine Myology. 

Also unsurprisingly, Brian Shaw is right about finding flap in Mexican meat markets. 

BDL