You can find Flat Iron Steaks at Fred Meyer/Kroger. They come in a vaccuum packed type packaging in the meat department. They will last in the fridge for a couple of weeks and much longer in the feezer. This is an awesome steak for the grill (best if done medium or medium rare) we have done it with a teriyaki style marinades or as simple as salt & pepper (w/ a hint of "jamaica-me-crazy" or johnny's seasoning) and best of all it's very inexpensive.
Hope this helps :)
Each HEB carries different cuts of beef (and for different prices). I forget which stores regularly carry flatiron but I know I've seen them more than once each at Hancock and Parmer stores.
A better cut to look out for is chuck eye. It's a steal lately at $3.49/lb. Often the HEB Natural choice brand is priced less than the USDA select version.
Flat iron steak is also known as top blade steak or top boneless shoulder steak; these terms are more likely to be found in stores. I have never seen a steak in a grocery store called a "flat iron" steak. Wikipedia says that flat iron steak is also known as: top blade roast, shoulder top blade roast, top boneless chuck, petite steak, butler steak, lifter steak, book steak, chuck clod, lifter roast, and triangle roast.
You may actually need to go to the meat counter and have them cut a blade roast for you. There aren't many flat iron steaks on the shelves, and when they are, they are pretty weak.
Whole foods also carrries them, and I am sure Central Market would too.
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