On my monthly Costco shop, I always buy a package of flap meat. It makes the best steak for the best price ($8-9 per pound - used to be $6!). It's a splurge but we get two "fancy" meals out of our $25 purchase. It comes in two pieces, so I put them in the freezer individually in gallon bags (I usually thaw by submersing the meat - still in the sealed bag - in a bowl of hot water for about twenty minutes).
To make this delicious steak, I heat up butter (or bacon fat) in our cast iron pan on medium high, turn on the fan, and cook it on one side for 5-7 minutes. I flip it and repeat. Sometimes I have to flip it one more time to get it perfectly medium to medium rare. I let it sit for a few minutes before slicing against the grain.
If I am cooking onions (usually red), I cook them in the (greased) pan for about 5-10 minutes and then add the steak, cooking them both together until the onions are nicely charred (I take them out before the steak). Sometimes I add mushrooms, but if I do that I usually cook it all before I make the steak, so as not to overcrowd the pan and because mushrooms are liquidy.
This is one of our favorite family dinners - steak, mashed potatoes, and salad (the kids don't have the onions and blue cheese - more for us!).